Visual Basic 6.0 Notes short

Notes For Visual Basic 6.0

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Introduction to Visual Basic 6.0 Visual basic is an ideal programming language for developing sophisticated professional applications for Microsoft windows. It makes use of graphical user interface for creating robust and powerful applications. Coding in GUI environment is easy and quicker as compare to traditional, linear programming languages. Visual Basic was developed from BASIC programming language. It required at least Microsoft windows 95/NT 3.51, 486 processor and minimum of 16 M.B. of RAM and also 250 MB of hard disk to install complete enterprise edition.

Starting Visual Basic 6.0 Visual Basic is initiated by using the Program option -> Microsoft Visual Studio and Microsoft Visual Basic 6.0 from it. Then it opens into a screen as shown below (fig 1.1) Fig .1.1

The integrated Development Environment One of the most significant changes in visual basic 6.0 is the integrated Development Environment (IDE). IDE is the term commonly used in the programming world to describe the interface and environment that we use to create our applications. It is called integrated because we can access virtually all of the development tools that we need from one screen called an interface. The visual basic IDE is made up of a number of components • Menu Bar • Tool Bar • Project Explorer • Properties Window • Form layout Window • Toolbox • Form Designer • Object Browser

Menu Bar

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This is bar display the commands that are required to build an application. The main menu items have sub menu items that can be chosen when needed.

Tool Bar The toolbar in the menu bar provide quick access to the commonly used commands.

Toolbox The toolbox contains a set of controls that are used to place on form at design time thereby creating the user interface area. Additional controls can be included in the toolbox by using the Components menu item of the Project menu. A toolbox is represented in fig 1.2

Pointer Label

Picture Textbox CommandButton OptionButton

Frame Checkbox

Listbox

Combobox HScrollBar Timer DirListBox

VScrollBar DriveListBox FileListBox Line Data

Shape Image OLE Fig 1.2 • • • • • • • • • • • •

The pointer provides a way to move and resize the control and forms. Label display a text that the user cannot modify or interact with. Frame control serves as a visual and functional container for control. Checkbox display a True/False or yes/no option. Textbox is a control used to display message and enter text. The Listbox display a list of items from which a user can select one. ComboBox contains a textbox and Listbox. This allows the user to select an item from Dropdown Listbox, or to type in a selection in the textbox. HScrollBar and VScrollBar controls allow the user to select a value within the specified range of value. Timer control executes the timer events at specified intervals of time. Dirlistbox allows the user to select the directories and paths, which are displayed Shape control use to draw shape on form. Image control is used to display icons, bitmaps, metafiles etc

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Notes For Visual Basic 6.0 • • • • • • •

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OLE control is used to link or embed an object, display and manipulate data from other windows based applications. Picturebox displays icons/bmp/jpg and metafiles. It displays text or acts as a visual container for other controls. CommandButton control which is a part of an option group allows the user to select only one option even if it display multiple choices. The FileListBox display a set of files from which user can select the desired one. The DriveListBox display the valid disk drives and allows the user to select one of them. Line control to draw a line on form. Data control enables the user to connect to an existing database and display information from it.

Project Explorer Docked on the right side of the screen, just under the toolbar, is the project Explorer window. It display objects of your project like forms, classes and modules. All the object that make up the application are packed in a project.

Fig 1.3 (project explorer )

Properties Window The properties window is docked under the project explorer window. The properties windows exposes the various characteristics of selected objects.

Fig 1.4 ( Properties window )

Form Layout The form layout window use to place the form starting position after you runs it.

Fig 1.5 (Form layout window)

Variables, Data Types and Modules Variable are used for storing values temporarily. Declaring a variable tells Visual basic to reserve space in memory. It is not must that a variable should be declared before using it. A defined naming strategy has to be followed while naming a variable. 1. A variable name must begin with alphabet letter and should not exceed 255 characters. 3


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2. It must be unique within the same scope. 3. It should not contain any special character like %,&, #, @ or $ Syntax Dim variable [as type] For e.g Dim str as string Dim I as integer

Scope of variables A variable is scoped to a procedure-level(local) or module-level depending on how it is declared. A variable is declared in general declaration section of a Form, and hence is available to all the procedures. Local variable are recognized only in the procedure in which they declared. They can be declaring with dim and static keyword.

Local Variable:

Local variable is one that is declared inside a procedure. This variable is only available to the code inside the procedure and can be declared using dim statement as given below Dim I as integer

Static Variables Static variables are not reinitialized each time visual basic invokes a procedure and thus retains or preserves value even when a procedure ends. e.g. Static in as integer

Module Level Variables Module level variables are available to all the procedure in the module. They are declared using Public or private keyword. e.g. public nk as integer private st as string declaring a variable with public keyword makes it available throughout the application even for the other modules. A variable can have same name with different scope.

Data Type: By default Visual Basic variables are of variant data types. The variant data type can store numeric, date/time or string data. When a variable is declared, a data is supplied for it that determines the kind of data they can store. A list of visual basic data types are given below Type Name Byte Boolean Integer Long Single Double Date

Values Range 0 to 255 True or False -32,768 to 32,767 -2,147,483,648 to 2,147,483,647 -3.402823 * 103 to -1.401298 * 10 45 for negative values. 1.401298 * 10-45 to 3.402823 * 10 38 for positive values. -1.79 * 10 308 to -4.94 * 10 -324 for negative values 4.94 * 10 -324 to 1.79 * 10 308 for positive values January 1, 100 to December 31, 9999 4

Notes For Visual Basic 6.0 String Currency

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0 to approximately 2 billion characters -922,337,203,685,477.5808 to 922,337,203,685,477.5807

Data Type Conversion Visual Basic functions either to convert a string into an integer or vice versa and many more conversion functions. A complete listing of all the conversion functions offered by visual basic is elucidated below. Conversion To Boolean Byte Currency Date Decimals Double Integer Long Single String Variant Error

Function Cbool Cbyte Ccur Cdate Cdec Cdbl Cint Clng Cn=sng Cstr Cvar CVErr

Note: A conversion function should always be placed at the right hand side of the calculation statement.

Procedure: Visual basic programs can be broken into smaller logical components called procedures. Procedures are useful for condensing repeated operations such as the frequently used calculations, text and controls manipulation etc. The benefits of using procedures in programming are: • It is easier to debug a program with procedures, which breaks a program into discrete logical limits. • Procedures used in one program can act as building block for other programs with slight modification. A procedure can be sub, function or property procedure

Sub Procedure A sub procedure can be placed in standard, class and form modules. Each time the procedure is called, the statement between Sub and End Sub are executed. Syntax [private | public ] [ static] Sub Procedure_Name [ (arg_list)] [Statements] End Sub Argument list is separated by commas. Each argument acts like a variable in the procedure. There are two type of sub procedures namely general procedures and event procedures.

Event Procedures An event procedure is procedure block that contains the control’s actual name, an underscore ( _ ), and the event name Syntax 5


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Private sub Form1_Load( ) Statement block End sub This is event for form1. Execute when load form.

General Procedures A general procedure is declare when several event procedure perform the same action. It is good programming practice to write common statements in separate procedure and then call them in the event procedure

Function Procedures Function are like sub procedures, except they return a value to calling procedure. They are especially useful for taking one or more pieces of data, called arguments and performing some tasks with them. Then function return value that indicate result of the task. e.g. The following function procedure calculates area of circle of given radius. Function Area(a as double) as double Area = 3.14 * a^2 End Function

Control Flow Statements Control statements are used to control the flow of program’s execution. Visual Basic supports control structures such as If -- --- -- Then, If -- --- -- Then -- -- -- Else-- End If. Select----Case and Loop structures, such as Do While-------Loop, Do-----Loop while and For…..Next method.

If….Then….Else statement The If… then block is used for conditional execution of one or more statements. Syntax: If Condition Then Statements End If The statement is execute only if the given condition is true. The condition is usually a comparison, but it can be any expression that evaluates a numeric value

If….Then …. Else Statement To execute one block of code if condition is false. Syntax: If Condition Then Statements Else Statements End If

Select ……..Case Statement Select…Case structure is an alternative to If …Then ….Else If for selectively execute a single block of statement from among multiple blocks. Syntax 6


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Select Case Index Case 0 Statements Case 1 Statements Case n Statements End Select Select …. Case statement evaluates an expression once at the top of the block where as if…then else structure evaluates different expressions for each Else If statement.

Do While…..Loop statements The Do While …. Loop is used to execute statements until a certain condition is met. Following do while .. loop counts from 1 to 100 Count=1 Do while count < 100 Count = count + 1 Loop

Do…Loop While Statement The do…..loop While Statement first execute the statements and then test the condition after each execution. The following program block is same to first one(like Do While loop) Count = 200 Do Text1.text = str(count) Count=count + 1 Loop While count < 300 The program execute the statements between do and loop while structure in any case. Then it determines weather the count is less than 300. If so, the program again execute block until it gets false.

Do….Loop Until statement The do….loop until structure execute the statements until the condition is satisfied. It is an infinite loop if the test fails and to get released from this loop we can use CTRL + BREAK combination or End From Run menu. The following code entered in the Form Load() procedure illustrates the structure. Private Sub Form_load() Do X$ = INputBox$(“Password ?”) Loop until X$ = “Sunil” End Sub

The For….Next Loop statement The for…Next Loop is another way to make loops in visual basic Syntax For Var_name = Start_val to End_val [ Step Number] 7


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Statements Next [Var_name] For I = 1 to 100 step 1 Text1.text = str(I) Next I

Exit For and Exit Do Statement A For…Next Loop Can be terminated by an Exit For Statement. Consider the following statement block For I = 1 to 100 Text1.text = str(i) If I= 50 then Exit For End if Next In this example loop is for 1 to 100 but we stop or exit from loop by using exit for statement in it. Same we can exit from do loop I=1 Do while I < 100 Text1.text = str (I) I = I+ 2 If I > 50 then Exit Do End If Loop

With ….. End With Statement When properties are set for objects or methods are called, a lot of coding is included that acts on the same object. It is easier to read the body by implementing with With… End With statement. For e.g suppose we want to set properties for Form then With form1 .backcolor = qbcolor(2) .enabled = True .MaxButton = false .Font = “Times” End with In the above coding, the object form get all its properties which we set in with block.

Modules Code in visual basic is stored in the form of modules. The three kinds of modules are Form module, Standard module, and class modules. A simple application may contain a single form and the code resides in that Form module itself. As the application grows, additional Forms are added and there may be a common code be to executed in several Forms. To avoid the duplication of code, a separate module containing a procedure is created that implements the common code. This is a standard module. Class module is a foundation of the object oriented programming in visual basic. New objects can be created by writing code in class modules. Each module can contain variable declaration or procedures.

Array in Visual Basic

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A sequence of variables by the same name can be referred using array. The individual elements of an array identified using an index. Arrays have upper and lower bounds and elements have to lie within those bounds. Each index number in an array is allocated individual memory space. We can declare array of type variant, user defined types and object variables.

There are two types of array • •

Fixed size array: The size of array always remains the same Dynamic array: The size of can be changed

Fixed Size Array Fixed array can be declared by giving a name with the upper limit in the parentheses. The long data type. e.g dim lengths(10) as Integer In the above illustration, lengths is a name of the array and the number 10 include in the parentheses is the upper limit of the array. This create array having 10 elements with index number from 0 to 9. Dim length(1 to 10) as Integer having lower limit 1 to upper index number 10. Dim marks(50,50) it create two dimensional array. Dim marks(1 to 10,1 to 10) it create two dimensional array with lower and upper limits.

Dynamic array There will be a situation when the user may not know the exact size of the array at design time. Under such situation, a dynamic array can be initially declared and can add elements when needed instead of declaring the size of the array at design time. Dim newarray( ) The actual elements can be allocate using ReDim statement ReDim newarray( n ) Each time on the executing the ReDim statement, the current data of array is lost and the default value is set. But if we want to change the size of array without lost any previous elements then use keyword Preserve ReDim Preserve newarray(20)

User-Define Data Type Variable of different data types when combined as a single variable to hold several related information is called user-define data type. A type statement is used to create new type. User-define type can only be private in form while in standard modules can be public or private. e.g. Private type student Name as string Age as integer Per as double End type e.g. Creating variables Dim s1 as student e.g. Or creating array 9


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Dim s(10) as student Accessing members of user-define type Variable name.member name e.g. text1.text = s1.name accessing name of student s1

Visual Basic Built-in Functions Many built-in functions are offered by Visual Basic that fall under various categories. • Date function • Format functions • String functions

Date Functions Now Year() Month() Day() WeekDay() Hour() Minute() Second()

Return current system date and time. Return year number from given date. Return month number from given date. Return day of given date. Return week day number from given date. Return hour number form given time. Return minute from given time return second from given time.

DateAdd() Return a date to which specific interval has been added. Syntax: DateAdd(interval,number,Date) Where interval is “M” ,”D” or “Y” Number to add in date Date in which you want to add. e.g. DateAdd(“m”,3,”02/14/2002”) it return 06/02/2002 (add 3 month in date)

DateDiff() Returns a Variant (Long) specifying the number of time intervals between two specified dates. Syntax DateDiff(interval, date1, date2[, firstdayofweek[, firstweekofyear]]) The DateDiff function syntax has these named arguments: Part

Description

interval

Required. String expression that is the interval of time you use to calculate the difference between date1 and date2.

date1, date2

Required; Variant (Date). Two dates you want to use in the calculation.

firstdayofweek

Optional. A constant that specifies the first day of the week. If not specified, Sunday is assumed.

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Notes For Visual Basic 6.0 firstweekofyear

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Optional. A constant that specifies the first week of the year. If not specified, the first week is assumed to be the week in which January 1 occurs.

Dim TheDate As Date ' Declare variables. Dim Msg TheDate = InputBox("Enter a date") Msg = "Days from today: " & DateDiff("d", Now, TheDate) MsgBox Msg DatePart()

Returns an integer containing the specified part of the given date

DateValue()

Convert a string into date format.

String Functions StrComp() Compare two strings Lcase() Convert the given string into lower case. Ucase() Convert the given string into Upper case. Len() Return a length of given string Format() Convert a given string into given format. Lset(),Rset() Justify a string Instr() Return a variable(long) specifying the position of the first occurrence of the string within another. Letf() Return specified number of character from left side of given string Rigth() Return specified number of character from Right side of given string Trim() Return a given string by removing all it’s blank space from left and right side. Mid() Return a specified number of character from mid of given string. Ltrim()Trim all blanks of left hand side. Rtrim() Trim all blanks of Right hand side.

Working with Forms In visual basic, the form is the container for all the controls that makes up the user interface. A form may fill entire screen or have other forms within it. It may be a custom dialog box. When a Visual basic application is executing, each window it displays on the Desktop is Form.

Setting Form Properties Border Style None Fixed single Sizable Fixed Double Fixed ToolWindow Sizable Toolwindow

No border is shown. A single-pixel width border is shown This is default setting. A double-pixel width is shown around the form. this type of border is use for toolbars. Same as fixed toolwindow, but border are resizable.

Caption The title for the window is store in the caption property. Control Box This is determine weather the control box, available by clicking the upper left corner /icon of a window. Icon This property specify the icon for the window in the upper-left corner of the window 11


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MousePointer This property sets the value that indicate the type of mouse pointer displayed. MaxButton/MinButton Set max or min button of window which are present on title bar. Moveable By setting it false it is possible to prevent the user from moving window. MDIChild This property specifies if this window must be shown within a multiple document interface (MDI) window. StartUpPosition Set the starting position of window.

Displaying Forms Show Method The show method is used to display the form object. For e.g. to display the form frmcalculator, the following code is written. Frmcalculator.show Using Load Statement The load statement is used to load a form or control into memory but does not display it. To load the form frmcalculator, the following code should be enter. Hiding and Unloading Forms Using the Hide Method The hide method hides a form. For e.g., the following code hides the form object frmcalculator Frmcalculator.Hide Using the Unload Statement The unload statement removes the form from the display and releases the memory. The following code should be entered to unload frmcalculator from memory. Unload frmcalculator

Setting Run time and design time properties Many properties can be set at either design time or runtime. At design time, the controls are added to a project, their properties set and relevant code is written. To set properties at design time following steps are followed. 1. Select the control or object whose properties you want to set. 2. Activate the property window 3. Scroll to the property you want to set 4. Enter a new setting. At run time properties can change by program’s code. This is done by Object.propertyname = value e.g Private Sub Form_Load() 12


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Form1.backcolor = QBcolor(3) Form1.Caption = “Sample Application” Form1.WindowStatus = VBNormal Form1.Width = 5000 Form1.Height = 5000 End Sub

Creating and Using Controls A control is an object that can be drawn on a Form object to enable or enhance user interaction with an application. Controls Classification of controls Visual Basic Controls are broadly classified as standard controls, Active X controls and insert-able objects. Standard controls such as CommandButton, Label and frame controls are contained inside .EXE file and are always include in the ToolBox which cannot be removed. ActiveX controls exist as separate files with either .VBX or .OCX extension. They include specialized controls, few from them given below • MSChart control • The Communication control • The Animation control • TreeView control • The Picture clip control • SysInfo control

Text Box Control A TextBox control, some times called an edit field or edit control, display information entered time at design time, entered by the user at run time or assign any value at run time. Some Properties of a Text Box Name This is a name used in program to identify the control MaxLength Set maximum no of characters that can be entered in control. MultiLine weather a control can accept multiple lines or not. PasswordChar It replace the text of control with given character with. Text to assign text to control at design time.

Command Button A command Button control to begin, interrupt, or end a process. When chosen, a Command Button appears pushed in and so is sometimes called push button. Properties of a command button control To display text on a command button control, set its caption property. An event can be activated by clicking a command button A button’s background color can be set by setting Backcolor property. Text color of a command button can be set using forecolor property. Font can be change using font property. Enabled or disable button Tooltip text can be set using Tooltip property. A button click event is handled whenever a command button is clicked. To add a click event handler, double click on button at design time, which adds a subroutine like the one given below. Private sub Command1_Click() ..... . . . 13


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End Sub

Using option button control Option button a set of choices from which a user can select only one button by • Clicking at run time. • Assigning the value property to True in code like Option1.Value = True • Using the shortcut keys specified in the caption of a label To disable Option button at run time its Enabled property set to False.

Using Listbox and ComboBox Controls ListBox and ComboBox controls present a set of choices that are displayed vertically in a single column. If the number of items exceed the value that can be display, scroll bars will automatically appear on the control. These scroll bars can be scrolled up and down or left to right through the list.

Adding items to a list It is possible to populate the list at design or at run time. Design Time: To add items to a list at design time, click list in the property box and then add the items. Press CTRL + ENTER after adding each item. Run Time: The AddItem method is used to add items to a list at run time. The AddItem method uses the following syntax: Object.AddItem item, Index The item argument is string that represents the text to add to the list. The index argument is an integer that indicates where in the list to add the new item.

Removing Items from a List The RemoveItem method is used to remove an item from a list. The syntax for this is given below. Object.RemoveItem.index

Selecting an Item from a list To access the items in a listbox we use the ListIndex and List properties. The list index property sets the index number of the currently selected item. The first item in the list having index number 0. ListIndex return -1 if no item is selected. The list count return total number of items in listbox. Sorting the List To sort the list set sorted property to True.

Using ComboBox A Combo Box combines the features of Textbox. This control enables the user to select either by typing text into combo box or by selecting an item from list. There are three types of Combo Box styles that are given below. Dropdown Combo (style 0) Simple Combo (style 1) Dropdown list (style 2)

Using ScrollBar Control The Scrollbar is a commonly used control, which enables the user to select a value by positioning it at the desired location. It represents a set of values. The min max property represents the minimum and maximum value. The value property represents its current value. 14


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Managing Menus Creating and managing menu at Design Time Visual Basic applications can be enhanced by adding menus to it. It offers a convenient and consistent way to group commands and easy way for users to access them. Title Bar Menu Bar

Separator bar

The menu bar appears appear below the title bar and it may contain one or more menu titles. When a menu title is clicked it displays a set of menu items under that title. Each menu item corresponds to a menu control that is defined in a menu editor and performs a specific action.

Using the Menu Editor A menu editor can be used to add new commands to the existing bars. A menu editor can be added only after opening a project. To display a menu editor command is chosen from the Tools menu or menu editor button is clicked in the tool bar. Writing Code form menu Control Each menu control has click event, which is execute when the menu item is selected or clicked. The following code is entered in the code window for green color option of menu. Private sub grn_Click() Form1.backcolor= QBcolor(2) Grn.Enabled = False Blue.Enabled = True Red.Enabled = True End sub When the green color is selected by clicking on it, form backcolor will be change into green color and that green option form menu is make disable and to other’s enable. Adding a Separator Bar and Shortcut key. Separator bar is a line that separates the menu items, which is mainly useful for obtaining clarity. To add it in menu, Right click on the form select menu editor Place the pointer where you want to insert separator In the caption TextBox ‘-‘ is typed and give any name 15


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Using Access and Shortcut keys Access keys allow the user to open a menu by pressing the Alt key with a letter key. To open the Edit menu in all Windows applications, for e.g. you can press Alt-E. . Access key is designed by the programmer and they are marked with an underline character. To assign Access key to menu item, add & sign before letter. e.g. “&File”. Shortcut Keys Shortcut keys are similar to access key has a special meaning in menu design, but inside a opening menu, they run a command when pressed. To assign a shortcut key to a menu command, dropdown the shortcut list in the menu editor and select a keystroke.

Pop-Up Menus A pop-up menu is a floating menu that is displayed over a form independent of the menu bar. Pop-up menus are also called context menus. Because the items displayed on the pop-up menu depend on where the pointer is located when the right mouse button is clicked. Any menu can be displayed as a pop-up menu at run time provided it has one menu item. The following code displays the colors menu when the user clicks right mouse button over the form at run time. Private Sub Form_MouseDown (Button As Integer, Shift As Integer, X As Single, Y As Single) If Button = 2 Then PopupMenu cc End If End Sub

Making menu control invisible It run time if user wants to enable and disable to menu items, it is possible by setting the Enabled property to True or False. When a menu item is disabled, it is dimmed and cannot be selected. But it is still visible and the menu items seen. Mnu_green.Enable = False But if you want to hide menu item from display set visible property false for it. Mnu_green.Visbile = False Using Check Marks In some programs we may require check marks to be placed in the menu items. To place a check mark in a menu item, the checked property is set to true. The following example code entered in the Form_load( ) procedure places a check mark in the menu item Red. Mnured.checked = True

Menu Control Array: A menu control array is a set of menu items on a menu that share the name and event procedure. Each menu control array element is identified by a unique index value, indicated in the Index property box on the menu editor. When the member of the menu control array recognizes an event, visual basic passes the index property value to the event procedure as additional argument. The event procedure must include the code that can check the value of the index property.

Mouse Events Visual basic applications respond to various mouse events, which are recognized by most of the controls. The main events are MouseDown, MouseUp and MouseMove. MouseDown occurs when the user 16


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presses any mouse button and MouseUp occurs when the user releases any mouse button. MouseMove occurs whenever the mouse pointer is moved to a new point on the screen. These events use the arguments button, Shift, X, Y and they contain information about the Mouse’s condition when the button is clicked. The first argument is an integer number called button. The value is 1 if left button is clicked and 2 if right button is clicked. The second argument is an integer called shift. The value of this argument indicates weather the mouse button was clicked simultaneously with the Shift key or Alt key. The third and fourth arguments X and Y are the coordinates of the mouse location at the mouse button was clicked

e. g. This is program to draw a line on form with the help of mouse. First method the starting points, when you press mouse button down to draw line (MouseDown events) Private Sub Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single) Form1.CurrentX = X Form1.CurrentY = Y End Sub

Second method for drawing a line on current points (Mouse/Move events) Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single) If Button = 1 Then Line (Form1.CurrentX, Form1.CurrentY)-(X, Y) End If End Sub

Dragging and dropping Dragging is a process of clicking the mouse button in a control and moving the mouse while holding down the mouse button. The action of releasing the mouse button after the dragging is called dropping. The following drag-and –drop properties, events and method are supported in Visual Basic. DragMode property enables automatic or manual dragging of a control. DragIcon property specifies the icon that is display when the control is dragged. DragDrop event is recognized when a control is dragged onto the object. DragOver event is recognized when a control is dragged over the object. Drag method starts or stops manual dragging. All the controls except menus, timer, lines and shapes support the above mentioned properties and method. Forms recognize the DragDrop and DragOver events but they do not support Drag method and the properties DragMode and DragIcon. To develop an application to explain drag and drop events we use a image, textbox and command button

Private Sub Command1_DragOver(Source As Control, X As Single, Y As Single, State As Integer) Dim inf As String info = "NOW DRAGGING " info = info + img.Tag

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info = info + " OVER THE Exit button" info = info + " STATE" + Str(State) Text1.Text = info End Sub Private Sub Form_DragDrop(Source As Control, X As Single, Y As Single) Text1.Text = "" img.Move X, Y End Sub Private Sub Form_DragOver(Source As Control, X As Single, Y As Single, State As Integer) Dim inf As String info = "NOW DRAGGING " info = info + img.Tag info = info + " OVER THE FORM " info = info + " STATE" + Str(State) Text1.Text = info End Sub

In this program when user drag image icon on form or command button related message will be display on the screen, and when release the mouse the image will be place or move on the current location of mouse pointer.

File Controls Usually you use the Common Dialog File Open and File Save As to get file names and file paths from the user, but sometimes that just won’t do. For e.g. you have a program where you want to let user select files but don’t want to use dialog boxes, In that and similar cases, you can use the visual basic file controls: The Drive List Box, Directory List Box and File list Box Drive List Box: Use this tool to draw a drive list box in a form, you get currently selected drive in a drive list by using its Drive property, and when the user changes the drive in the control, a Change event is generated. But File controls always work with each other. e.g.

Drive List box Directory List box File List box Text Box RichText Box

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Here in e.g when the user selects a new drive, we pass that new drive on ot a directory list box, dir1 using that drive as the new root directory in Dir1: Sub Drive1_Change() Dir1.Path = Drive1.Drive End Sub Using The Directory List Box The directory list box control displays directories as a hierarchical set of folders. The important property of the list box is Path property, which holds the path of the current directory. When the user changes the current path, a Change events is generated. For example, when the user makes a change in directory list box, dir1, we can pass the new path to file list box, File1 Sub Dir1_Change() File.Path = Dir1.Path End Sub Using The File List Box Control The file list box control lets you display the files in a directory as a list of names. The important properties of the File list box are Path and FileName properties. In our e.g. we load file select from File List box to Picture Box and txt file in RichText Box. So when we click on File name following event get execute. Private Sub File1_Click() On Error Resume Next Dim fname As String fname = File1.Path + "\" + File1.FileName Text1.Text = fname If LCase(File1.Pattern) = "*.bmp" Or LCase(File1.Pattern) = "*.gif" _ Or LCase(File1.Pattern) = "*.ico" Then Picture1.Picture = LoadPicture(fname) ElseIf LCase(File1.Pattern) = "*.txt" Then rtb.FileName = fname End If End Sub

Graphics for Application Visual Basic provides a variety of ways to create and use graphics in an application, which adds styles, interest and visual structure to the interface of an application. Graphic objects such as lines, circles and bitmaps can be displayed in visual basic in a quicker and easier way.

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The number of times a twip is used by all visual basic movements, sizing and graphical drawing statements is limited to one. A twip is a unit that specifies the dimensions and location of the graphics objects. There are 1440 twips in one inch. Thses measurements designate the size when printed. The coordinate system is a two dimensional grid that defines the location on the form or any other container which is represented as(X, Y). X represents the location of a point on the X-axis and Y represents a point on the Y-axis A color is represented by long integer and there are four ways of specifying it at run time. They are specified using RGB function, QBColor function, and using one of the intrinsic constants listed in the Object Browser and by entering a color value directly RGB(255,0,0) return Red color RGB(0,255,0) return Green color RGB(0,0,255) return Red color QBcolor function takes a single number that specifies a Quick Base color number from 0 to 15 and returns a long integer that can be used in visual basic color property. Form1.backcolor = QBColor(4)

Using Graphics Controls Visual Basic provides three controls to create graphical applications such as image, Line and shape. These controls are very useful at design time. The main advantage of graphics controls is that we can create an application with less code.

Line Control A line control is a straight-line segment that is drawn at design time. The position, length, color and style of the line control can be positioned to customize the look of an application

Shape Control A shape control is a Visual element that contains several predefined shapes. In order to view a specified shape, the control is added to the form by double clicking it. The default shape will be rectangle. The shape property is selected from the properties window, which drops down a list of shapes, from which the user can select the desired one. The FillColor, FillStyle properties of the Shape control can be changed so that desigred color and style can be obtained.

Image Control An image control is a rectangular portion into which picture files can be loaded. The picture files include bitmap files, icon files and metafiles. Adding Pictures: Line control and shape control are used for drawing geometrics shapes such as lines, circles, squares and so on. For drawing more complex figure we can use a picture file. A picture file can be loaded on a form, image control or Picture Control. A picture can be added by the following two ways at design time. • In the properties window of the Form, the Picture property is selected. Visual Basic displays a dialog box from which a picture file can be selected. Similarly a picture can be loaded in a PictureBox and Image control. • A picture can be paste on form or into PictureBox or Image control A picture can be added by the following ways at run time. • LoadPicture function is used to specify the filename and assign the picture to the Picture property. Picture1.Picture = LoaPicture(“C:\Image\sst.jpg”) 20

Notes For Visual Basic 6.0 • •

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Any picture loaded to a form, PictureBox or Image control can be copied to another Form, PictureBox or Image control. Set picture1.picture = Image1.Picture A picture can be copied from ClipBoard object

Removing Picture A picture can be removed at run time using LoadPicture function without arguments. Set image1.picture = LoadPicture(“ “) Moving and Sizing Pictures If a form, PictureBox or an Image control is moved, the picture associated with it also moves automatically. The autosize property of a picture box can be set to True in order to automatically expand and accommodate a new picture. Using Graphics Methods Every graphics method draws output on a Form, PictueBox or to the printer object. The following statement draw a point on the form Form1.PSet(500,500) The PSet method draw a point at x, y co-ordinates that is specified by its arguments. Form1.Cls Clear all the graphics PixelColor = point(30,40) Point method return the color of a particular pixel. Line(x1, y1) – (x2, y2), Color Line method draw a line from x1,y1 to x2,y2 of given color, x1 ,y1 is a starting point coordinates and x2, y2 is ending points. Visual Basic provides a much simpler way to draw a box. Line(600,600) – (1000,1000), Qbcolor(4),B B option is used with Line Method to draw a box. A Circle method is used to draw a variety of circular and elliptical shapes. To draw a circle Visual Basic required the location of the circle’s center and length of its redius. Circle(1400, 1200), 500

Multiple Document Interface (MDI) MDI stand for Multiple Document Interface. A Multiple Document Interface is used for opening many Windows at the same time. All the document windows are contained in a parent window, which provides a workspace in the application. Visual Basic applications can have only one MDI form, which contains all the child forms. A child form is an ordinary Form that has its Child property set True. Child form are display within the internal area of MDI form at run time. Add new MDI form to Project: To add new MDI form, select it from project menu or right click on project name in Explorer window and select Add, MDI form. 21


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To all other forms of your project set MDIChild property True to associate these forms with parent form. e.g. In this example we create a MDI form having two child form Name Form1 and Form2 after clicking on Load form Option of Form menu, show form1 and on form2 show form2. window use to show list of open form Steps: 1. Open New project 2. Add MDI form from project menu 3. Add Form from project menu 4. To form1 and Form2 set MDIChild property to True 5. Create the menu Form Load Form1 Load Form2 Exit Window Enter following code in MDI form Private Sub loadfrm1_Click () Form1.Show End Sub Private Sub loadfrm2_Click () Form2.Show End Sub

Debugging in Visual Basic Debugging is a process by which error are identified and resolved in source code. In visual basic error can occur anywhere during entire development cycle such as • Design time • Compile time • Run Time

Design Time Bugs Errors that occurs in the IDE and before the program is compiled are called design-time bugs. These are the most common ones. These bugs are caused by misuse of some component. The following e.g. shows a function that works as long as a textbox control is passed to it. If any other type of control (say text box, here, the extra space causes the problem) is passed, it fails to execute and an error is reported this is a design time error, which is display the moment the user navigates to the next statement. Sub settext( ctrl as text box, txt as String) Ctrl.text = Lcase(Trim$(txt) + “ : ” + Date$ End sub On entering the first statement of the above coding in declaration section, the following error is report.

Compile-time bugs

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Compile-time bugs are those that occur when we attempt to create the program executable file (EXE) or run the project. Visual Basic can locate compile-time bugs if the Visual Basic application is set up correctly. Visual Basic set the several points to users, which can be change as needed. Compile-time bugs are detected by visual basic automatically when the program is compiled using F5. If the Start With Full Compile option is used, the Compile On Demand can be turned on, and the developer will not find any bugs until the line of code with the bugs is actually executed. Compile errors occur as a result of incorrectly constructed code such as Next statement without a corresponding For statement or programming mistakes that violate the rules of Basic, such as misplace word, missing separator, or type mismatch. Compile errors included syntax errors. These include mismatched parentheses or an incorrect number of arguments passed to an intrinsic function.

Runtime bugs A Visual Basic runtime error is the exception generated by Visual Basic when it ascertains that the code is about to perform something illegal. An illegal function could be something as simple as trying to determine the size of a file that dose not exist or attempting to multiply two numbers, the result of which exceeds the storage space that could be contained by the data type. However, using an error handler could prevent this error dialog box from being shown and save the program from crashing. By practices we can also use our knowledge that a particular error identified by its error number will be generated to help guide our logic. This sort of “inline” error handling is very powerful and can be handled using an error handler that checks for the condition of, say Error 57, and then does something to avoid the situation. This error can be finally rectified by changing the code suitably. E.g. Consider the following example where we declare two variable x and y. try dividing x (assign a value 9) by y (assign 0), assigning the quotient to another variable z.

Private sub Form1_load() Dim x as Integer Dim y as Integer Dim z as Integer X=9 Y=0 Z=x/y End Sub

Visual Basic Debugging Tools The best way to keep bugs out is to prevent then in the first place. Visual Basic gives a programming several tools, include IDE options and compile directives, to help achieve this goal.

IDE Options 23

Notes For Visual Basic 6.0 • • • • • • •

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Visual Basic offer several IDE option that can help the user write better code. Auto Syntax Check Required Variable Declaration Auto List Members Auto Quick Info Auto data Tips Option Explicit Option Compare Text

Additional settings and debugging aids can be viewed in a dialog box shown in the fig. using the following steps 1. Select Option from Tools Menu 2. Choose the Editor tab in the Option dialog box

Require Variable declaration: This option explicitly inserts the Option Explicit statement in each new form, module, or class created. A dim, private, public, ReDim or static statement must be used to declare a variable before the value is assigned to it. If an undeclared variable is used, an error occurs at compile time. e.g. Option Explicit Private sub Form1_Load() Sum = 0 End sub

Auto Syntax Check: The Auto Syntax Check the line of code when the user moves the cursor off the line.

option forces Visual Basic to check

e.g. Private sub Form1_load() Dim a integer End sub In this program Dim a here required “As” so compiler give a error message when we try to move cursor to next line.

Auto Quick Info: The Auto Quick Info in Visual Basic display ant required or Optional parameters for methods. VB display all possible parameters while highlighting the one we are currently entering. Auto Quick Info makes it easier to use methods and properties because we no longer have to remember or look up all the arguments it’s properties and methods. Debugging Tools are designed to help the user with logic and run-time errors. Visual Basic provides several buttons in the ToolBar that are helpful for debugging. They are Breakpoint: Defines a line in the code window where Visual Basic suspends execution of application Instant Watch: Lists the current value of an expression while the application is in the break-mode. Calls: Presents a dialog box that shows all procedures that have been called but not yet run completely.

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Visual Basic provides several built-in methods for controlling the execution of the program in real time. It is possible to execute the program line by line or procedure by procedure or a combination of the two. These basic debugging actions are called Stepping. Because it enables the developer to walk through the program, examining the variables and logic, stepping is the most powerful debugging tool offered by Visual Basic. The various debugging commands are discussed below. Step Into: Execute the next executable line of the code in the application and steps into procedures. This enables to check every line of code as it is being executed. This option can be accessed from debug menu or by pressing F8. Step Over: Execute the next executable line of the code in the application without stepping into procedure. The procedure the user steps over does execute, but Visual Basic does not walk through the stepped-over procedure line-by-line. Shortcut key is Shift+F8. Step Out: Moves program execution back to the calling procedure. This is the functional equivalent of Exit sub or Exit Function. It is simple exits the current procedure without executing any more code in that procedure. Set Next: Enables the user to move program execution to any executable line of code in the current procedure. While debugging an application, we should clearly understand which of the three modes such as design time, run time or break mode we are in at a given time. Break mode of an application is viewed by clicking CTRL+BREAK at run time.

Using the Debugging Window Debug window is one of Visual Basic’s window that allows to run individual commands immediately, by typing in the command and pressing the Enter key. It automatically opens at run time. 1. In break mode, the debug window can be used to execute individual line of code, View or change values of variables and properties, and view watch expressions. 2. At run time, it can be used to display data or messages as the programs as the program runs. 3. At design time, the developer can view the previous output to the Debug window, but cannot execute code. The debug window has two part- Watch window and immediate window. The split bar separates the Debug window into two panes. The upper pane display the Watch Window. The lower pane display the Immediate Window.

Watch Window Watch pane displays the current watch expressions, which are expressions whose values are decided by the user as code is executed. The Watch pane appears automatically if the watch expressions are defined in the project. At times we want to monitor the value of a variable for a certain state- for example to determine weather a flag is set to True or False. Steps to add watch expression 1. Add watch command is chosen from the Debug menu. 2. The Expression is entered in the Expression box. 3. If necessary, the scope of the variables is set and an option button is selected. 4. The OK button is clicked.

Immediate Window The immediate pane appears by default the first time the debug window is opened. From break mode, the code is executed immediately by entering it in this pane. This window is the right place for the 25


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users to modify data or test functions during development. We can enter any valid expression in this window and VB will execute it. If a reference is made to an object outside the scope of the current code execution, Visual Basic will generate an error. Testing data and procedure in Immediate Pane While debugging an application, Sometimes it may necessary to execute individual procedures, evaluate expressions or assign new values to the variables or properties. The Immediate Pane can be used to accomplish new values to the variables or properties. The Immediate Pane can be used to accomplish such tasks. Expressions can be evaluated by printing in Immediate Pane. Some advantages of these facility. • The data or message can be viewed at run time. • Feedback is displayed in separate area, so that it does not interfere with the output that the user is seeing. • Since the coding is saved as part of the form, these statements needs not be redefined the next time we work on the application. e.g. Private sub Form_load( ) Dim a as integer Dim b, c as integer A = 10 B= 6 C=a–b Msgbox “Value of c is “ & C Debug.print “ Value of c is :” & c End sub On executing the above code the value of c is displayed in the Immediate window using the Debug.print statement.

Using a Breakpoint to selectively Halt Execution At run time, a breakpoint tells VB to halt before executing a specific line of code. A breakpoint can be set or removed at design time or at break mode. To set or remove a breakpoint, The insertion point is moved to the line of code where the breakpoint is to be set or removed. The Toggle Breakpoint is chosen from the Debug Menu or Toggle button is chosen in the Toolbar or press shortcut key F9. Writing Error Handlers Visual Basic has specific built-in ways to handle runtime errors, called trappable errors. When such an error occurs, you can direct the execution of your program to an error handler, which is a section of code written specifically to deal with errors.

On Error GoTo Label. The visual basic On error Goto statement is the foundation of handling trappable errors. When you execute an On Error GoTo Label Statement in your code. Execution is transferred to the code starting at label if a trappable error has occurred. The code following that label is your error handler. Private Sub Form_Load() Dim a, b, c As Integer On Error GoTo diverror a = 10 b=0 c=a/b MsgBox "Value of C =: " & c diverror:

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MsgBox "Divided by Zero Error", vbCritical, "Error" End Sub

In this example value of b is zero, if don’t trap error then our program is terminate with an error, but in this we trap error and give appropriate Message to user.

Err : It is a object Contains information about run-time errors. When a run-time error occurs, the properties of the Err object are filled with information that uniquely identifies the error and information that can be used to handle it. To generate a run-time error in your code, use the Raise method. Main Properties of Err object Err.Number : Return error number Err.Description : Return error description e.g. you may show this error message in program. MsgBox "Des: " & Err.Description & "NUMBER : " &

Err.Number

On Error GoTo line# Besides using a label to start an error handler, you can refer to an error handler by line number in visual basic, using on error goto line# statement. e.g. 1 2 3 4 5 6 7 8 9

Private Sub Form_Load() Dim a, b, c As Integer On Error GoTo 8 a = 10 b=0 c=a/b MsgBox "Value of C =: " & c MsgBox "Divided by Zero Error", vbCritical, "Error" End Sub

Using On Error Resume Next The On Error Resume Next statement provides an easy way to disregard errors, if you want to do so. Once you execute this statement, execution continues with the next line of code if the current line generates an error, and the error is disregarded.

On Error GoTo 0 To turn off error trapping, you can use the On Error GoTo 0 statement.

Err Function Returns a Variant of subtype Error containing an error number specified by the user. Syntax CVErr(error number) The required errornumber argument is any valid error number. Remarks Use the CVErr function to create user-defined errors in user-created procedures. For example, if you create a function that accepts several arguments and normally returns a string, you can have your function evaluate the input arguments to ensure they are within acceptable range. If they are not, it is likely your

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function will not return what you expect. In this event, CVErr allows you to return an error number that tells you what action to take. Note that implicit conversion of an Error is not allowed. For example, you can't directly assign the return value of CVErr to a variable that is not a Variant. However, you can perform an explicit conversion (using CInt, CDbl, and so on) of the value returned by CVErr and assign that to a variable of the appropriate data type. e.g Sub Test() Debug.Print CalculateDouble("345.45robert") End Sub ' Define CalculateDouble Function procedure. Function CalculateDouble(Number) If IsNumeric(Number) Then CalculateDouble = Number * 2 ' Return result. Else CalculateDouble = CVErr(2001) ' Return a user-defined error End If ' number. End Function

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Database: Using DAO, RDO, and ADO You may wonder why there are three different sets of database object in visual basic, the reason is historical. At first, VB only support DAO, which connected to the Microsoft Jet database engine (the database engine in MS Access). Then, recognizing that there are other database types available, Microsoft created the Open database connectivity (ODBC) standard and supported ODBC with Remote Data Objects in Visual Basic. Finally For Web to access data over network Microsoft create Active Data Object (ADO) that allows connections on the same computer, over networks and through the web, and is intended to support ODBC. ADO is also called OLE DB, and in fact, it’s based on COM programming techniques. There are two ways to work with DAO, RDO and ADO object sets in VB. The first way is working with the special controls that support them. The Data Control, Remote Data Control and ADO data control. You use them to connect to and to move through database but they don’t display actual data, you bind them to other Visual Basic controls, and those bound controls handle the display. The second way is working with the three database object sets directly in code, without controls like the data control or DAO data control, and that’s what we’ll do in the next chapter.

What are Databases? Nearly all business applications need to store large volumes of data, organized in a format that simplifies retrieval. This is accomplished with a database management system (DBMS), a mechanism for manipulating tabular with high-level commands. The database management system hides low-level details, such as how data are store in a database, and frees the programmers to concentrate on managing information, rather than on the specifics of manipulating files or maintaining links among them. Each individual data entry in a table, such as a student’s name, goes into a field in the table. A collection of fields together, such as the Name and Grade fields in our table, make up a record. Each record gets its own row in a table, and each column in that row represents a different field. A collection of records- that is, row of records where each column is a field-become a table. What’s then is a Database? A database is just a collection of one or more tables.

Evolution of computing Architectures Let’s us discuss about the evolution of client-server architecture for various computing models. In this chapter we discuss about three architectures.

Centralized System Architecture Companies that needed real computing power turned to the mainframe computer, which is centralized system architecture. The salient most feature of a centralized system is that the only movement of marshalling are keystrokes between the client and the host machine. Marshalling is the process of packing interface elements and sending them across process boundaries. Thus in a centralized system, keystrokes are marshalling from the terminal (client) to the host. The centralized system architecture is illustrated in fig.

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Application Data

Data

Network Server Keystrokes Terminal Character

Terminal1

Terminal1

Terminal1

Terminal1

Merits of Centralized System Architecture • Excellent Security • Centralized administration as both application logic and data reside on the same machine De-Merits of Centralized System Architecture • Mainframe computers are very expensive to buy, lease, maintain and use. • The limitation is that both the application and the database live within the same machine process thereby offering no way to truly partition the application logic beyond the physical limitations of mainframe. In 1980 personal computers made an entry into the business world along with which, came the resources like printers, modems and hard-disk storage. The introduction of the local area network(LAN) and the use of file server architectures soon following the introduction of personal computer. File Server Architecture The file server system brought a complete change in implementation of the computer architecture from the mainframe. In this system, the application logic now executed on the client workstation instead of the server. These servers also provided access to computing resources like printer and large hard disk. The complete File Server architecture is illustrated in fig. The merit of the file server system is the low cost entry point with flexible arrangement. Computer resources can be added or reduced as and when necessary using this system. The drawback of the file server architecture is that all application logic is executed on the client machine. The job of the server is to provide files only to store the data. Though the application’s file might be located on the server, the application runs in the client machine’s memory space using the client’s processor. This results in the client machine’s need memory amount of power to run the application. Taking into account the de-merits of the centralized and file server system architecture, the client server architecture made its advent.

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Data Network Server Disk I/O required

Disk Blocks Terminal1

Terminal2

Terminal3

Terminal4

App. Logic1

App. Logic2

App. Logic3

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Client Server Model Client server architecture is a process involving a minimum of two independent entities, one is the client and the other is the server. The client makes a request to the server and the server services the requests and the resulting data is sent to the client. In this application, two separate applications operating independently could work together to complete a task. A well brought out implementation of this concept is the SQL based Database Management System. The client server architecture is well illustrated in fig. here, unlike the file server system, the request that goes to server in not merely a request for file in the form of disk input/output requested as series of disk blocks. Instead, instructions are communicated to an application running on the server, and the server executes those requests and sends a response to the client.

Network Server Disk I/O required

Data Application Logic

Disk Terminal1

Terminal2

Terminal3

Terminal4

App. Logic1

App. Logic2

App. Logic3

App. Logic4

The request may take the form of the SQL query submitted to a SQL database engine. The database engine in turn processes the request and a result set is send to the client. Thus the two independent process work together to accomplish a task and exemplifies the client-server relationship. The most popular client/server applications revolve round the use of DBMS such as Oracle and MS SQL server. These applications are referred to as backend and offer support for storage manipulation and retrieval of business persistent data. They use structure query language (SQL). As a standard method for submitting client requests. 31


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Thus a major advantage of Client/Server architecture is reducing traffic and, in most cases, an incredibly quicker execution time.

Two Tire and Three Tire Client Server Model The various system architecture that has been in use have been discussed so far. Now let us move on to discuss the creation of in information system that can change with business needs, needs that mandate tighter budget and higher quality in addition to simply processing transactions generating report. To respond to the challenges being presented by the business environment a new three-tier or n-tier client/server approach has been introduced.

Two Tier Client/Server model In two-tier model, a desktop machine operates as a client and network server function as a back-end database engine. The logic in two in two-tier model is split between the two physical location namely the server and client. The client in a two-tier model is necessarily a 4GL like Visual Basic or Power Builder. The salient point for a two-tier application is that the business logic must physically reside either on the client or be implemented on the back-end within the DBMS in the form of triggers and stored procedures. Both triggers and procedures are stored as precompiled collections of SQL statements and control-of-flow statements. In VB, using any of the data controls that provide a graphical link to the back-end data source create a two tier client/server relationship.

Merits of two-tier The merits of this two-tier model are – data access is simplified, allowing very rapid development of applications. The GUI is bound directly to the data source and all the details of data manipulation are handled automatically. But there is a disadvantage in this, i.e. though the data access is simplified, it is less flexible due to which the user will not have complete control of interactions with the data source.

Limitations of Two-Tier model Not Scalable: Inability of two-tier approach to grow beyond the physical boundaries of a client and server machine. Unmanageable: Business rules cannot be encapsulated and deployed centrally because of which sharing common process and reusing work become difficult. Poor Performance: The graphical interface binding to the data consumes major resources on the client machine, which results in poor performance and dissatisfied clients.

Three-Tier Client/Server Model: An improved model for client/server development resulted as an outcome of limited effectiveness of two-tiered client/server solutions. The three-tier client/server model is based on the ability to build partitioned applications. Partitioning an application breaks our code into logical components. The Service Model employed to design this architecture suggests that these components can be logically grouped into three tier. User services, Business services and Data services. The service and its location are given below. Service User Services Business logic Data services

Location Client Server Server

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On developing an application using this model and technique, each component can then be placed on a suitable machine that will provide the best performance depending on our situation and the current business need. The three-tier logic can be extended to any number of tier/services called as Multi-Tier or n-Tier architecture.

Values of three-tier client/server development Reuse: The time invested in designing and implementing components is not wasted as they can be shared among applications. Performance: As components can be placed on machines other than the client workstation, load processing can be shifted from a client machine that may be underpowered to a server with extra horsepower. This offers us the best possible methods for each aspect of our application’s execution, resulting in better performance. Manageability Encapsulation of applications services into components allows us to break down large, complex applications into more manageable pieces. Maintenance: centralizing components for reuse has an added benefit. They become easier to redeploy on making any amendments to keep pace with business needs.

Data Access Options Visual Basic provides a variety of options to access remote Client/Server databases. They are: Data Access Object (DAO): It communicates with the data source through the JET database engine. Data Control: It binds data-aware control to Microsoft Access and other ODBC data sources. ODBCDirect: It allows accessing of ODBC data source through the RDO with ADO objects, bypassing JET database engine. Remote Data Objects (RDO): It provides a framework for using code to create and manipulate components of a remote ODBC database system. Remote Data Control (RDC): It binds the controls to an ODBC remote database. Open Database Connectivity (ODBC): this is an API call interface to the open Database Connectivity libraries and drivers to provide data access to Microsoft SQL server and other database that provide an ODBC driver. Visual Basic Library for SQL Server (VBSQL): it is an implementation of the DB library API specifically designed to provide access to SQL Server through a visual basic application Active Data Object (ADO): This is a programming model that eliminates the need to choose from among DAO and RDO and all other data access methods. It is designed to provide a common bridge between different database, files systems and e-mail servers.

Data Access Objects A DAO is a collection of object classes that model the structure of a relational database system. They provide properties and methods that allow to accomplish all the operations to manage such a system, including features for creating databases, defining tables, navigating and querying a database. Visual Basic supports Data Access Object such as DBEngine, Workspace, Database, TableDef, Field, Index and Recordset Objects. 33

Notes For Visual Basic 6.0 • •

• • • •

• • • •

•

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DBEngine: The DBEngine is the top level database object and corresponding to the Jet database engine. This object is used to set database engine system parameters and default workspace. Workspace: The Workspace object is used to support simultaneous transactions and it acts as a container for open database. A default Workspace object Workspace (0) is created when the DAO are referenced in the language at run time. The default workspace is always available and never be closed or removed from the collection. Database: The Database object corresponds to a Jet native or external database or a direct ODBC connection. This is used to define the database’s table, relations and stored queries and to open Recordset object. TableDef: The TableDef object corresponds to stored table definition. Each TableDef in a collection represents the Definition of a current table in a database or an attached table in the external database. QueryDef: The QueryDef is a stored query definition, which is a precompiled SOL statement. RecordSet: The Recordset object corresponds to a cursored view into a database table or the results of query. A cursored view is that stores rows of data in buffer and points to one row of data at a time called current record. The cursor may be positioned to any row of data using Move, Seek or find methods. Field: The field object corresponds to a column of data type and set of properties. TableDef, Recordset objects have collection of field objects. The collection of Field object associated with Recordset cursor describes a single row of data. Index: The index object is a stored index allows to quickly reorder the records in a table. Parameter: A parameter object represents a parameter associated with a QueryDef object created from a parameter query. A parameter’s collection contains all the parameter objects of a QueryDef object. User: The User object is used to define and enforce database security. The DBEngine object supports a set of user collection. The Users collection contains all stored User objects supports a collection of system groups. Each user in the group inherits the permissions to access the objects that the group can access. Relation: A Relation object represents a relationship between fields in the tables or queries. A Relations collection contains stored Relation objects of Database object. The DBEngine enforces certain update and delete conditions on the data associated with the fields of the Relation object, to maintain referential integrity. Property: A property object represents a built-in characteristic or user-define characteristic of a data access object. Properties collection contain all the property object for a specific instance of an object.

Accessing and Navigating Databases In order to work with data objects, a reference has to be set to the appropriate DAO library. There are two DAO libraries supported by VB they are: • Microsoft DAO 3.51 Object library • Microsoft DAO 2.5/3.51 Compatibility Layer. To set reference to DAO type library: • Select References from the Project Menu • Select the DAO 3.51 (or 3.6) Object Library • Click on OK button

Opening Database To open an existing database, the OpenDatabase method of workspace object is used Syntax Opendatabase(dbname,[option],[readonly],[connect]) 34


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The following code opens the employee_detail database Dim db as Database Set db = opendatabase(“C:\Emp_detail.mdb”) In the above code, db is a variable that represents the Database object. By default, a database that is opened can be shared and modified by any user. To specify that the database is to be opened fro exclusive use, the following statement can be used: Set db = opendatabase(“Emp_detail.mdb”,True) In the above code True value indicates that no users will be able to open the database. The default value is False To open emp_detail database in the readonly mode, the following statement is used: Set db = Opendatabase(“Emp_dateil”,False,True) In the above statement, the True value specified as the third argument will provide only a read access on the database.

Recordset: A recordset is an object that contains a set of records from the database. There are mainly five types of Recordset object. 1. Table-Type Recordset: The table type recordset object is a set of records that represents a single table can be used to add, change or delete records. They are fastest type of Recordset. 2. Dynaset-Type Recordset: The dynaset-type recordset object is a set of records that represent a table, or attachment tables, or the results of queries containing fields from one or more tables. A dynaset enables us to update data from more than one table. 3. Snapshot-Type Recordset: the snapshot type recordset can refer any table, attached table or query. A snapshot cannot be updated and does not reflect changes to data made by the users. 4. Dynamic type Recordset: This recordset type represent a query result set from one or more base tables in which we can add, change or delete records from a row returning query. Further, records that other users add, delete, or edit in the base tables also appear in our recordset. This type is only available in ODBCDirect workspace, and corresponds to an ODBC dynamic cursor. 5. Forward Only Type recordset: This recordset type is identical to a snapshot except that we can only scroll forward through its records. This improves performance in situations where only need to make a single pass through a result set. In an ODBCDirect workspace, this type corresponds to an ODBC forward-only cursor.

Creating a Recordset: The OpenRecordset method is used to open a Recordset and create a recordset variable. To create a read-only recordset for the table emp, the following code is used. Dim rs as Recordset rs = db.OpenRecordset(“emp”,dbopentable,dbReadOnly) In the above statement, db is the variable that represents the database object. Here dbOpenTable specifies the type of the Recordset to be created.

Navigating a Recordset: •

After creating a Recordset object, the various methods can be used to navigate through the recordset. The MoveFirst :mehthod moves to the first row in the Recordset 35

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The MoveLast :mehthod moves to the Last row in the Recordset The MoveNext :mehthod moves to the Next row in the Recordset The MovePrevious :mehthod moves to the Previous row in the Recordset

Using BOF and EOF to navigate through Recordset The Recordset object provides two properties for the user to know when he has moved to the beginning or end of the recordset The EOF (End of File) property is True when the user moves beyond the last record in the recordset. The BOF (Beginning of file) property is true when the user moved to a position before the first record in the recordset.

Modifying and Deleting Records To manipulate a recordset, the following methods are used. Edit Method: The user can edit the current record using the edit method. The Update method is used to save the necessary changes made to the records. AddNew Method: AddNew method is used to add a blank record in the recordset. Delete Method: This method can be used to delete record in the dynaset-type recordset. The jet engine deletes the current record without any warning when the Delete method used. Finding Records The Find methods can be used to locate a record in dynaset-type or snapshot-type Recordset. VB supports four Find methods. FindFirst: method finds the first record satisfying the specified criteria. FindLast: method finds the Last record satisfying the specified criteria. FindNext: method finds the next record satisfying the specified criteria, searching backward from the current record. FindPrevious: method finds the previous record satisfying the specified criteria, searching backward from the current record.

Open Database Connectivity (ODBC) Open database connectivity (ODBC) is a windows technology that lets a database client application connect to external database. To use ODBC, database vendor must provide an ODBC driver for data access. Once this driver is available, the client machine should be configured with this driver.

Creating ODBC Data Source Before any application can access an ODBC database, the ODBC driver must be installed and a Data source name (DSN) created using the control panel. The DSN is used whenever a reference is made to an ODBC database. Follow these steps to create ODBC for Access 1. Double click on ODBC configuration option under control panel. A list of ODBC data source is displayed as shown in fig. 2. Select Add button to add new ODBC drivers that are currently installed in system are displayed as shown in fig.2

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3. Select Microsoft Access Driver (*.mdb) from list and click on Finish button. 4. Type any Data source name and select database for which you want to create DSN by clicking on Select button. 5. Click on OK button to complete process.

Using ODBC with DAO The properties and methods of DAO can be used in conjunction with ODBC database. OpenDatabase The OpenDatabase method creates a connection between the application and the ODBC database and assigns it to a database type object. Syntax Dim db as Database Set db = opendatabase (“”, , “” ODBC; UID=; PWD=”)

The Active Data Objects Visual Basic supports several data access tools, with the Active Data Object (ADO) being the most recent addition. Whereas the first Visual Basic data access tools (the Data Access Object, discussed before this) allowed programmers to access Access database only, ADO can access all major databases and Microsoft’s foundation for a universal technology for accessing all types of data in all environments. With ADO, your VB application sees three objects: 1. A connection object, which establishes a connection to the database, be it a local file or remote SQL Server. 2. A command object, which executes commands against the database. 3. A Recordset Object, which holds the records retrieved from the database or the records to be updated on the database. As we saw AtiveX Data Objects (ADO) access data from OLE DB providers. The Connection object is used to specify a particular provider and any parameters. To connect to a data source, you use a Connection object. Using that connection, you can create a new record set, and using the recordset object’s methods and properties, you can work with your data. An ADO transaction marks the beginning and end of series of data operations that are executed across a connection. You can cancel or allow changes.

ADO Example To explain ADO data handling in code, we’ll build an ADO project. This application lets you open the db1.mdb file of access using ADO objects to edit records, add, delete records. You can move through the database using arrow buttons you see in fig. To edit a record, just type the new value into the text box and click on update button. To add new record, use add button and insert new values, after it click on update button. ADO: Opening a Connection 37


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The first step to start the ADO project is give reference to Microsoft ActiveX Data Object library item, and click on OK to add object library to your project Then create object of Connection and Recordset Dim Cn As New ADODB.Connection Dim Rs As New ADODB.Recordset Now we are free to create a new ADO connection to object with the Connection object’s Open method: Connction.Open ConnectionString[, UserID [, Password [, OpenOption]]] Here are the arguments for this method. Connection String-String containing connection information. UserID- String containing a username to use when establishing the connection Password – String containing a password to use when establishing the connection. OpenOptions - If set adConnectAsync, the connection will be opened asynchronously. Code Private sub form_load() dim cn as new ADODB.Connection cn.open “PROVIDER = Microsoft.Jet.OLEDB.3.51;Data source = “C:\db1.mdb;” …. …. End sub Creating a Recordset from Connection Now that’s you have created an ADO connection, you open a record set from that connection using the Recordset object’s Open method Recordset.Open [ Source, [ Active_connection , [Type, [ LockType, [ options]]]] Here the arguments for this method Source- A valid command object variable name, an SQL statement, a table name, a stored procedure call, or the file name of Recordset. ActiveConnection- A valid Connection object variable name or a string containing ConnectionString parameter. Type- Sets the recordset type (see the following list) • dbopenKeyset: Opens a dynaset-type recordset object, which is like an ODBC keyset cursor. • dbOpenDynamic: Opens a dynamic-type Recordset object, which lets the application see changes made by other • dbOpenStatic: Open a static type recordset object. • dbOpenForwardOnly: Opens a forward-only-type Recordset object, where you can only use MoveNext to move. LockType: A value what determines what type of locking the provider should use when opening the record set. Here are the possible values for the LockType: adLockReadOnly: the default; readonly. adLockPessimistic: Pessimistic locking, record by record. adLockOptimistic: Optimistic locking, record by record. 38


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adLockBatchOptimistic: Optimistic batch updates. e.g. Private sub form_load() dim cn as new ADODB.Connection dim rs as new ADODB.Recordset cn.open “PROVIDER = Microsoft.Jet.OLEDB.3.51;Data source = “C:\db1.mdb;” rs.Open “Select * from lib”, cn , adOpenStatic,adLockOptimistic …. End sub ADO: Binding Controls to Record sets To bind a control to ADO Recordset objects, you just set that controls DataSource property to that object, and then set whatever other data properties that control needs to have set e.g. Private sub Form_load() Dim cn as new ADODB.Connection Dim rs as new ADODB.Recordset cn.open “PROVIDER = Microsoft.Jet.OLEDB.3.51; Data source = “C:\db1.mdb;” rs.Open “Select * from lib”, cn , adOpenStatic,adLockOptimistic set text1.datasource = adorecordset text1.Datafield = “Title” set text2.datasource = adorecordset text2.Datafield = “cost” End sub ADO: Adding a Record to a Recordset To add a new record to an ADO recordset, you use the AddNew method, after you’ve updated the fields of the current record, you save that record to the database with the update method. here’s how you use AddNew: Recordset.AddNew [ Fields [, Values]] Fields: a single name or an array of names or ordinal positions of the fields in the new record. Values: A single value or an array of values for the fields in the new record. If Fields is an array, values must also be an array with the same number of members. e.g. rs.AddNew ADO: Updating A Records in a Record Set. After changing the data in a record’s in fields or adding a new record, you update the data source to record the changes, using the Update method: Recordset.Update fields,values Here are the arguments for this method: Fields - A single name or an array of names or ordinal positions of the fields in the new record. 39


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Values - A single value or an array of values for the fields in the new record. If fields is an array, Values must also be an array with the same number of members. e.g. rs.Update ADO: Navigating a Recordset: After creating a Recordset object, the various methods can be used to navigate through the recordset. • The MoveFirst : Method moves to the first row in the Recordset • The MoveLast : Method moves to the Last row in the Recordset • The MoveNext : Method moves to the Next row in the Recordset • The MovePrevious : Method moves to the Previous row in the Recordset Using BOF and EOF to navigate through Recordset The Recordset object provides two properties for the user to know when he has moved to the beginning or end of the recordset The EOF (End of File) property is True when the user moves beyond the last record in the recordset. The BOF (Beginning of file) property is true when the user moved to a position before the first record in the recordset. Modifying and Deleting Records To manipulate a recordset, the following methods are used. Edit Method: The user can edit the current record using the edit method. The Update method is used to save the necessary changes made to the records. AddNew Method: AddNew method is used to add a blank record in the recordset. Delete Method: This method can be used to delete record in the dynaset-type recordset. The Jet-engine deletes the current record without any warning when the Delete method used.

Input Validation What is Input Validation? Input validation is the process of checking the data entered by the user before that data saved to the database. This is some times referred to as Client-side validation. This is because the validation action happens at client’s workstation. Common Input Validation Rules Almost every field in your database required some type of input validation. Before you design form, put together a list of all fields you need on the form and answer the following questions. • Must data be entered in the field? (Is it required field?). • What characters are valid/invalid for this field (numeric input only, capital letters only, on space allowed, and so on)? • For numeric field is there high/low range limit? Solution • If possible, limit the keystrokes to valid values only. For example is the field must be numeric, don’t allowed the user to enter character values. If space are not allowed, make sure spacebar is disabled. • •

Limit input choices with lists. If there is a limited set of valid inputs for a field, give the user a pick list or set of radio buttons to choose from. Input the user of range limits. If the field has the low and high limit, tell the user what the limits are. 40

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Point out required fields on a form. Mark required fields with a leading asterisk (*) or some other appropriate character.

Field level Validation The first level of validation is at the field level. This is the place where you can make sure the user is entering the right character in the field, entering the data into the field in proper format, and entering a valid value based on a list of possible choices. Discarding Unwanted Keystrokes Limit data entry in the Key press events Private sub text1_KeyPress (Keyascii As Integer) Dim str as string Str = “0123456789.” If Instr(str,chr(keyascii) ) = 0 then Keyascii = 0 End if End sub You declare a string that holds the list of valid keys. The next line loads the string with the valid key fields, and the next line checks to see weather the key pressed is in the string of valid keys. It does this by converting the numeric value passed by Visual Basic in the keyascii parameter (the ASCII value of key Pressed) into readable character using function chr( ). To convert the input characters into uppercase Private Sub text1_Keypress(KeyAscii As Integer) KeyAscii = Asc(Ucase(chr(KeyAscii))) ‘change to upper case End sub

Input Masking It is very common to have fields on your form that required special input formats. Examples of special formats are telephone numbers, government or employee identification numbers, hour / minute time entry, and so on. VB 6 ships with a bound data control that handles special input and display formatting: the MaskedEdit control. The MaskedEdit control works like the standard VB 6 textbox control, with few added properties that make it a powerful tool for your input validation. To add MaskedEdit control, first select the project, components and then click on Browse button, scroll down the list to MSMASK32.OCX select it and click on OK button. The Masked Edit control provides restricted data input as well as formatted data output. This control supplies visual cues about the type of data being entered or displayed. This is what the control looks like as an icon in the Toolbox: File Name MSMASK32.OCX

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Class Name MaskEdBox Properties Format: Specifies the format for displaying and printing numbers, dates, times, and text. Mask: Determines the input mask for the control. Syntax [form.]MaskedEdit.Mask [ = string$] Remarks: You can define input masks at both design time and run time. However, the following are examples of standard input masks that you may want to use at design time. The control can distinguish between numeric and alphabetic characters for validation, but cannot check for valid content, such as the correct month or time of day. Mask

Description

Null String

(Default) No mask. Acts like a standard text box.

##-???-##

Medium date (US). Example: 20-May-92

##-##-##

Short date (US). Example: 05-20-92

##:## ??

Medium time. Example: 05:36 AM

##:##

Short time. Example: 17:23

The input mask can consist of the following characters. Mask character

Description

#

Digit placeholder.

.

Decimal placeholder. The actual character used is the one specified as the decimal placeholder in your international settings. This character is treated as a literal for masking purposes.

,

Thousands separator. The actual character used is the one specified as the thousands separator in your international settings. This character is treated as a literal for masking purposes.

:

Time separator. The actual character used is the one specified as the time separator in your international settings. This character is treated as a literal for masking purposes.

/

Date separator. The actual character used is the one specified as the date separator in your international settings. This character is treated as a literal for masking purposes.

\

Treat the next character in the mask string as a literal. This allows you to include the '#', '&', 'A', and '?' characters in the mask. This character is treated as a literal for masking purposes.

&

Character placeholder. Valid values for this placeholder are ANSI characters in the following

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ranges: 32-126 and 128-255. >

Convert all the characters that follow to uppercase.

<

Convert all the characters that follow to lowercase.

A

Alphanumeric character placeholder (entry required). For example: a – z, A – Z, or 0 – 9.

A

Alphanumeric character placeholder (entry optional).

9

Digit placeholder (entry optional). For example: 0 – 9.

C

Character or space placeholder (entry optional). This operates exactly like the & placeholder, and ensures compatibility with Microsoft Access.

?

Letter placeholder. For example: a – z or A – Z.

Literal

All other symbols are displayed as literals; that is, as themselves.

Prompt: Sets or returns the character used to prompt a user for input. Syntax [form.]MaskedEdit.PromptChar [ = char$] Remarks: The underscore character "_" is the default character value for the property. The PromptChar property can only be set to exactly one character. Note: To clear the Text property when you have a mask defined, you first need to set the Mask property to an empty string, and then the Text property to an empty string: MaskedEdit1.Mask = "" MaskedEdit1.Text = ""

Form Level Validation : Form level validation is an essential part of designing a good validation scheme for your form. Although many input errors can be caught and corrected at the field level, there are several validation steps that can be performed well only at the form level. The form level validation is performed at the time when the user presses Enter, or clicks the OK or Save button. These are validations that are done after the user has entered all the fields, but before any attempt is made to store the values to a table.

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