C# Language Combine Delegates (Multicast Delegates)


Example

Addition + and subtraction - operations can be used to combine delegate instances. The delegate contains a list of the assigned delegates.

using System;
using System.Reflection;
using System.Reflection.Emit;

namespace DelegatesExample {
    class MainClass {
        private delegate void MyDelegate(int a);

        private static void PrintInt(int a) {
            Console.WriteLine(a);
        }

        private static void PrintType<T>(T a) {
            Console.WriteLine(a.GetType());
        }

        public static void Main (string[] args)
        {
            MyDelegate d1 = PrintInt;
            MyDelegate d2 = PrintType;

            // Output:
            // 1
            d1(1);

            // Output:
            // System.Int32
            d2(1);

            MyDelegate d3 = d1 + d2;
            // Output:
            // 1
            // System.Int32
            d3(1);

            MyDelegate d4 = d3 - d2;
            // Output:
            // 1
            d4(1);

            // Output:
            // True
            Console.WriteLine(d1 == d4);
        }
    }
}

In this example d3 is a combination of d1 and d2 delegates, so when called the program outputs both 1 and System.Int32 strings.


Combining delegates with non void return types:

If a multicast delegate has a nonvoid return type, the caller receives the return value from the last method to be invoked. The preceding methods are still called, but their return values are discarded.

    class Program
    {
        public delegate int Transformer(int x);

        static void Main(string[] args)
        {
            Transformer t = Square;
            t += Cube;
            Console.WriteLine(t(2));  // O/P 8 
        }

        static int Square(int x) { return x * x; }

        static int Cube(int x) { return x*x*x; }
    }

t(2) will call first Square and then Cube. The return value of Square is discarded and return value of the last method i.e. Cube is retained.