14.2. A first delegate example

Let's combine what we have done so far into a full concrete example. Study the program below.

 1:  using System;
 2:
 3:  delegate int MyDelegate (int i);
 4:
 5:  class TestClass{
 6:
 7:    static int Double (int val){
 8:      Console.WriteLine("running Double");
 9:      return val*2;
10:    }
11:
12:    int Triple (int val){
13:      Console.WriteLine("running Triple");
14:      return val*3;
15:    }
16:
17:    public static void Main(){
18:
19:      TestClass tc = new TestClass();
20:      MyDelegate d1, d2;
21:
22:      d1 = new MyDelegate(TestClass.Double);
23:      d2 = new MyDelegate(tc.Triple);
24:
25:      Console.WriteLine(d1(3));
26:      Console.WriteLine(" ---------- ");
27:      Console.WriteLine(d2(5));
28:    }
29:  }

Output:

c:expt>test
Running Double
6
----------
Running Triple
15

The MyDelegate type is declared on line 3 outside the scope of any class. MyDelegate is compatible with any method that returns an int, and takes in a single int as parameter. TestClass has two methods: a static Double, and a non-static Triple. Since both Double and Triple return an int, and take in a single int as parameter, both methods are compatible with the MyDelegate type. Two delegate instances of MyDelegate are created on lines 22 – 23 – d1 encapsulates the static method Double, while d2 encapsulates the instance method Triple.

On lines 25 and 27, both delegate instances are invoked with different int parameters. When d1 is invoked (line 25), the static method Double is invoked with 3 as the input parameter – the value of 6 is returned and printed out. When d2 is invoked (line 27), the instance method Triple is invoked with the parameter 5 – the value 15 is returned and printed out.