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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.
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.
There are four ways to declare the Main
procedure. It can take arguments or not, and it can return a value or not.
Nota
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
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
End If
' Insert call to appropriate starting place in your code.
MsgBox("The application is terminating.")
End Sub
End Module
.NET maklum balas
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Peristiwa
17 Mac, 9 PTG - 21 Mac, 10 PG
Sertai siri perjumpaan untuk membina penyelesaian AI berskala berdasarkan kes penggunaan dunia sebenar dengan rakan pembangun dan pakar.
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Introduction to .NET - Training
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