Main Procedure in Visual Basic

Artykuł
11/16/2012

Every Visual Basic application must contain a procedure called Main. This procedure serves as the starting point and overall control for your application. The .NET Framework calls your Main procedure when it has loaded your application and is ready to pass control to it. Unless you are creating a Windows Forms application, you must write the Main procedure for applications that run on their own.

Main contains the code that runs first. In Main, you can determine which form is to be loaded first when the program starts, find out if a copy of your application is already running on the system, establish a set of variables for your application, or open a database that the application requires.

Requirements for the Main Procedure

A file that runs on its own (usually with extension .exe) must contain a Main procedure. A library (for example with extension .dll) does not run on its own and does not require a Main procedure. The requirements for the different types of projects you can create are as follows:

Console applications run on their own, and you must supply at least one Main procedure. .
Windows Forms applications run on their own. However, the Visual Basic compiler automatically generates a Main procedure in such an application, and you do not need to write one.
Class libraries do not require a Main procedure. These include Windows Control Libraries and Web Control Libraries. Web applications are deployed as class libraries.

Declaring the Main Procedure

There are four ways to declare the Main procedure. It can take arguments or not, and it can return a value or not.

Note

If you declare Main in a class, you must use the Shared keyword. In a module, Main does not need to be Shared.

The simplest way is to declare a Sub procedure that does not take arguments or return a value.

Module mainModule
    Sub Main()
        MsgBox("The Main procedure is starting the application.")
        ' Insert call to appropriate starting place in your code.
        MsgBox("The application is terminating.")
    End Sub
End Module

Main can also return an Integer value, which the operating system uses as the exit code for your program. Other programs can test this code by examining the Windows ERRORLEVEL value. To return an exit code, you must declare Main as a Function procedure instead of a Sub procedure.

Module mainModule
    Function Main() As Integer
        MsgBox("The Main procedure is starting the application.")
        Dim returnValue As Integer = 0
        ' Insert call to appropriate starting place in your code.
        ' On return, assign appropriate value to returnValue.
        ' 0 usually means successful completion.
        MsgBox("The application is terminating with error level " _
            & CStr(returnValue) & ".")
        Return returnValue
    End Function
End Module

Main can also take a String array as an argument. Each string in the array contains one of the command-line arguments used to invoke your program. You can take different actions depending on their values.

Module mainModule
    Function Main(ByVal cmdArgs() As String) As Integer
        MsgBox("The Main procedure is starting the application.")
        Dim returnValue As Integer = 0
        ' See if there are any arguments.
        If cmdArgs.Length > 0 Then
            For argNum As Integer = 0 To UBound(cmdArgs, 1)
                ' Insert code to examine cmdArgs(argNum) and take
                ' appropriate action based on its value.
            Next argNum
        End If
        ' Insert call to appropriate starting place in your code.
        ' On return, assign appropriate value to returnValue.
        ' 0 usually means successful completion.
        MsgBox("The application is terminating with error level " _
            & CStr(returnValue) & ".")
        Return returnValue
    End Function
End Module

You can declare Main to examine the command-line arguments but not return an exit code, as follows.

Module mainModule
    Sub Main(ByVal cmdArgs() As String)
        MsgBox("The Main procedure is starting the application.")
        Dim returnValue As Integer = 0
        ' See if there are any arguments.
        If cmdArgs.Length > 0 Then
            For argNum As Integer = 0 To UBound(cmdArgs, 1)
                ' Insert code to examine cmdArgs(argNum) and take
                ' appropriate action based on its value.
            Next argNum
        End If
        ' Insert call to appropriate starting place in your code.
        MsgBox("The application is terminating."
    End Sub
End Module

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Main Procedure in Visual Basic

Requirements for the Main Procedure

Declaring the Main Procedure

See Also

Concepts

Reference

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