How to: Combine Delegates (Multicast Delegates)(C# Programming Guide)


Updated: July 20, 2015

System_CAPS_ICON_note.jpg Note

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This example demonstrates how to create multicast delegates. A useful property of delegate objects is that multiple objects can be assigned to one delegate instance by using the + operator. The multicast delegate contains a list of the assigned delegates. When the multicast delegate is called, it invokes the delegates in the list, in order. Only delegates of the same type can be combined.

The - operator can be used to remove a component delegate from a multicast delegate.

        using System;

        // Define a custom delegate that has a string parameter and returns void.
        delegate void CustomDel(string s);

        class TestClass
            // Define two methods that have the same signature as CustomDel.
            static void Hello(string s)
                System.Console.WriteLine("  Hello, {0}!", s);

            static void Goodbye(string s)
                System.Console.WriteLine("  Goodbye, {0}!", s);

            static void Main()
                // Declare instances of the custom delegate.
                CustomDel hiDel, byeDel, multiDel, multiMinusHiDel;

                // In this example, you can omit the custom delegate if you 
                // want to and use Action<string> instead.
                //Action<string> hiDel, byeDel, multiDel, multiMinusHiDel;

                // Create the delegate object hiDel that references the
                // method Hello.
                hiDel = Hello;

                // Create the delegate object byeDel that references the
                // method Goodbye.
                byeDel = Goodbye;

                // The two delegates, hiDel and byeDel, are combined to 
                // form multiDel. 
                multiDel = hiDel + byeDel;

                // Remove hiDel from the multicast delegate, leaving byeDel,
                // which calls only the method Goodbye.
                multiMinusHiDel = multiDel - hiDel;

                Console.WriteLine("Invoking delegate hiDel:");
                Console.WriteLine("Invoking delegate byeDel:");
                Console.WriteLine("Invoking delegate multiDel:");
                Console.WriteLine("Invoking delegate multiMinusHiDel:");
        /* Output:
        Invoking delegate hiDel:
          Hello, A!
        Invoking delegate byeDel:
          Goodbye, B!
        Invoking delegate multiDel:
          Hello, C!
          Goodbye, C!
        Invoking delegate multiMinusHiDel:
          Goodbye, D!

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