Creating Procedure Delegates

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 we'll see later in the book. 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.