Collections

provides the Collection class. The syntax for creating a collection is discussed as follows:

Dim CollectionName As New Collection()

In the preceding syntax, the name of the collection that you want to create is specified by CollectionName. An instance of the Collection class is created, which is declared by the New keyword in the declaration statement

After the creation of the collection, you can manipulate the creation in the same way as you would manipulate the standard collections that are provided by Visual Basic .NET. However, there are some differences between the two. Consider the following example:

Dim collection1 as New Collection()

collection1 = Controls

The preceding code creates and initializes a Collection object, collection1, with the Controls collection. However, this statement displays an error message. Why is it so? The answer is that the Controls collection and the Collection class object are not interchangeable and are of different types with different usage. In addition, they do not have the same methods and also do not use the same kinds of index values.

Procedures

Consider a scenario where you need to perform a particular task repeatedly, for instance, calculating the average of marks obtained by students in a particular subject. In a situation such as this, you can group them in a procedure instead of writing the statements repeated ly. A set of statements grouped together to perform a specific task is called procedure. You can organize your applications by using procedures that allow you to chunk and group the program code logically.

After grouping the statements in a procedure, you can call the procedure from anywhere in the application. To call a procedure means to execute a statement that further instructs the compiler to execute the procedure. After executing the code in the procedure, the statement following the statement that called the procedure is executed.The statement that is called by a procedure is called a calling statement, and it includes the name of the procedure.The calling statement also includes the

INTRODUCTION TO VISUAL BASIC .NET Appendix B 863

data values that are needed by the procedure for performing the tasks that are specified. The data values are also referred to as arguments or parameters.

Consider the example of calculating average mentioned previously. In this case, you can create a procedure that accepts the maximum and minimum marks obtained by students as data values and calculate the average. To call this procedure, the statement to be called must provide the minimum and maximum marks obtained by students as parameters.

Now consider some of the advantages that are offered by procedures. The first advantage is the reusability of code. In other words, a procedure can be created and used when it is required and if any statement has to be changed, you simply need to make the changes in a single location.This is useful mainly in the case of large and complex applications.The applications that use procedures are easier to debug. Additionally, you can easily trace the errors in a procedure without debugging the entire application code.

Now consider the scope or accessibility of procedures in an application. Similar to classes and variables, procedures have a scope. A procedure is generally declared in a class or a module. Therefore, you can call a procedure from the same class or module in which it is created. The scope of the procedure depends on the access modifiers that you use while the procedures are declared. The access modifiers supported by Visual Basic .NET are listed in Table B-3.

Table B-3 Access Modifiers for Procedures

Based on the functionality of procedures, they can be classified as:

Sub procedures. A sub procedure is used to perform a specific task.

Function procedures. A function procedure is used to perform the specific tasks and returns a value to the calling statement.

Property procedures. A property procedure is used to assign or access a value from an object.

Event-handling procedures. An event-handling procedure is used to perform a specific task when a particular event occurs.

Arguments

As stated earlier, variables, constants, or expressions are accepted as arguments by procedures. As a result, each time a procedure that accepts arguments is called, arguments need to be passed to the procedure. Based on the data values that are passed as arguments, the result can differ for each call to a procedure. Arguments can be passed to procedures by either value or reference.

Functions

Visual Basic .NET provides various built-in functions that can be used in applications. Some of the built-in functions are MsgBox, InputBox, CStr, DateDiff, and StrComp. The Microsoft.VisualBasic namespace contains a declaration for these built-in functions. These functions can be classified based on the tasks performed by the various built-in functions. The functions can be classified as follows:

Functions to enhance your programs are performed by the Application enhancement functions. Examples of Application enhancement func-

tions are MsgBox and InputBox.

Functions to manipulate strings are performed by String functions. Examples of String functions are StrComp, Len, and Trim.

Functions to manipulate date and time values are performed by the Date function. Examples of Date functions are DateDiff, Now, and Month.

Functions to convert one data type to another are performed by the Conversion function. Examples of Conversion functions are CStr, CDate, and Val.

INTRODUCTION TO VISUAL BASIC .NET Appendix B 867

Now, to add the functionality to the application, you need to write code for the button controls. After adding code to the button controls, the code for the application is as shown:

using System.Drawing; using System.Collections;

using System.ComponentModel; using System.Windows.Forms; using System.Data;

namespace SampleWindowsApplication

{

public class frmAcceptUserInput : System.Windows.Forms.Form

{

private System.Windows.Forms.Label lblUserName; private System.Windows.Forms.Label lblPassword; private System.Windows.Forms.Button btnSubmit; private System.Windows.Forms.Button btnExit; private System.Windows.Forms.TextBox txtUserName; private System.Windows.Forms.TextBox txtPassword;

private System.ComponentModel.Container components = null;

public frmAcceptUserInput()

{

InitializeComponent();

}

protected override void Dispose( bool disposing )

{

if( disposing )

{

if (components != null)

{

components.Dispose();

}

}

base.Dispose( disposing );

}

INTRODUCTION TO VISUAL BASIC .NET Appendix B 869

4.In the Templates: pane, select the Windows Application option.

5.In the Name: text box, type the name of the Windows application as

SampleWindowsApplication1.

6.Accept the default location as specified in the Location: text box. You may also choose to browse for the location where you want to save the application by clicking on the Browse button.

7.Click on the OK button to close the New Project dialog box.

Figure B-2 shows the New Project dialog box for the SampleWindowsApplication1 project in Visual Basic .NET.

FIGURE B-2 The New P roject dialog box for SampleWindowsApplication1

When you click on the OK button, Visual Studio .NET automatically creates the default files and a blank Windows form for you. Click on the Show All Files button in the Solution Explorer window to view a list of all the files created by Visual Studio .NET. Figure B-3 shows the default files and the blank form created by Visual Studio .NET.

FIGURE B-3 The default files and the blank form created by Visual Studio .NET

As you can see in Figure B-3, the blank form in Visual Basic .NET is created with an extension .vb. In addition, Visual Studio .NET creates some reference files for the SampleWindowsApplication1 project. Similar to Visual C# .NET, Visual Studio .NET creates a solution with the same name as that of the Windows application. Inside the solution, the project with the name SampleWindowsApplication1 is created.

Now, proceed with the creation of the SampleWindowsApplication1 application. To create the application, you need to add controls to the Windows form. Because Visual Basic .NET and Visual C# .NET are languages based on the .NET Framework, the IDE for the Visual Basic .NET applications is the same as that of the Visual C# .NET applications. The IDE for the Visual Basic .NET applications contains a toolbox that contains controls that you can use to create the Windows application. Figure B-4 shows the toolbox that contains standard controls for creating a Windows application in Visual Basic .NET.

INTRODUCTION TO VISUAL BASIC .NET Appendix B 871

FIGURE B-4 The toolbox for creating Windows application

From the toolbox, drag two label controls, two text box controls, and two button controls and place them on the form. Now in the Properties window of the controls, change the properties of the controls.

TIP

If the Properties window is not displayed, select the control and press the F4 key. Alternatively, you can select the Properties Window option on the View menu.

Change the following properties of the controls:

Form1

Name: Form1

Text: Accept User Input