By: Steven Holzner in VB.net Tutorials on 2010-11-17
Sometimes, it's useful to be able to pass the location of a procedure to other procedures. That location is the address of the procedure in memory, and it's used in VB .NET to create the callback procedures. To work with the address of procedures, you use delegates in VB .NET.
Here's an example; in this case, I'll create a delegate for a Sub procedure named DisplayMessage:
Module Module1 Sub Main() ⋮ End Sub Sub DisplayMessage(ByVal strText As String) System.Console.WriteLine(strText) End Sub End Module
I start by declaring the delegate type, which I'll call SubDelegate1, and creating a delegate called Messager:
Module Module1 Delegate Sub SubDelegate1(ByVal strText As String) Sub Main() Dim Messager As SubDelegate1 ⋮ End Sub Sub DisplayMessage(ByVal strText As String) System.Console.WriteLine(strText) End Sub End Module
Now I use the AddressOf operator to assign the address of DisplayMessage to Messager, and then use Messager's Invoke method to call DisplayMessage and display a message:
Module Module1 Delegate Sub SubDelegate1(ByVal strText As String) Sub Main() Dim Messager As SubDelegate1 Messager = AddressOf DisplayMessage Messager.Invoke("Hello from Visual Basic") End Sub Sub DisplayMessage(ByVal strText As String) System.Console.WriteLine(strText) End Sub End Module
And that's all it takes-this code will display the message "Hello from Visual Basic", as it should.
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