Programming Visual Basic .NET, Second Edition

Multicasting

At times it is desirable to multicast, or call two implementing methods through a single delegate. You accomplish multicasting by encapsulating the various methods in delegates. Then you combine the delegates using the Delegate.Combine( ) shared method. The Combine( ) method takes an array of delegates as a parameter and returns a new delegate that represents the combination of all the delegates in the array.

To see how this works, create a simplistic class that declares a delegate:

Public Class MyClassWithDelegate
    ' the delegate declaration
    Public Delegate Sub StringDelegate(ByVal s As String)
End Class

Then create a class (MyImplementingClass) that implements a number of methods that match the StringDelegate:

Public Class MyImplementingClass

    Public Shared Sub WriteString(ByVal s As String)
        Console.WriteLine("Writing string {0}", s)
    End Sub

    Public Shared Sub LogString(ByVal s As String)
        Console.WriteLine("Logging string {0}", s)
    End Sub

    Public Shared Sub TransmitString(ByVal s As String)
        Console.WriteLine("Transmitting string {0}", s)
    End Sub
End Class

Within the Run( ) method of your Tester class, you’ll instantiate three StringDelegate objects (Writer, Logger, Transmitter):

Dim Writer, Logger, Transmitter As MyClassWithDelegate.StringDelegate

You instantiate these delegates by passing in the address of the methods you wish to encapsulate:

Writer = New MyClassWithDelegate.StringDelegate( _ AddressOf MyImplementingClass.WriteString) Logger = New MyClassWithDelegate.StringDelegate( ...