C# Delegates simple example in .NET

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Delegates

A delegate in C# is similar to a function pointer in C or C++. Using a delegate allows the programmer to encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code which can call the referenced method, without having to know at compile time which method will be invoked.

A delegate will allow us to specify what the function we'll be calling looks like without having to specify which function to call. The declaration for a delegate looks just like the declaration for a function, except that in this case, we're declaring the signature of functions that this delegate can reference.
There are three steps in defining and using delegates:
  • Declaration
  • Instantiation
  • Invocation
A very basic example (SimpleDelegate1.cs):
using System;

namespace Akadia.BasicDelegate
{
    // Declaration
    public delegate void SimpleDelegate();

    class TestDelegate
    {
        public static void MyFunc()
        {
            Console.WriteLine("I was called by delegate ...");
        }

        public static void Main()
        {
            // Instantiation
            SimpleDelegate simpleDelegate = new SimpleDelegate(MyFunc);

            // Invocation
            simpleDelegate();
        }
    }
}
Compile an test:
# csc SimpleDelegate1.cs
# SimpleDelegate1.exe
I was called by delegate ...

Delegate Example
using System;
delegate int NumberChanger(int n);
namespace DelegateAppl {  
   class TestDelegate {
      static int num = 10;     
      public static int AddNum(int p) {
         num += p;
         return num;
      }
      public static int MultNum(int q) {
         num *= q;
         return num;
      }
      public static int getNum() {
         return num;
      }
      static void Main(string[] args) {
         //create delegate instances
         NumberChanger nc1 = new NumberChanger(AddNum);
         NumberChanger nc2 = new NumberChanger(MultNum);
        
         //calling the methods using the delegate objects
         nc1(25);
         Console.WriteLine("Value of Num: {0}", getNum());
         nc2(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −
Value of Num: 35
Value of Num: 175

Multicasting of a Delegate

Delegate objects can be composed using the "+" operator. A composed delegate calls the two delegates it was composed from. Only delegates of the same type can be composed. The "-" operator can be used to remove a component delegate from a composed delegate.
Using this property of delegates you can create an invocation list of methods that will be called when a delegate is invoked. This is called multicasting of a delegate. The following program demonstrates multicasting of a delegate
using System;
delegate int NumberChanger(int n);
namespace DelegateAppl {
   class TestDelegate {
      static int num = 10;
      public static int AddNum(int p) {
         num += p;
         return num;
      }
      public static int MultNum(int q) {
         num *= q;
         return num;
      }
      public static int getNum() {
         return num;
      }
      static void Main(string[] args) {
         //create delegate instances
         NumberChanger nc;
         NumberChanger nc1 = new NumberChanger(AddNum);
         NumberChanger nc2 = new NumberChanger(MultNum);
        
         nc = nc1;
         nc += nc2;
        
         //calling multicast
         nc(5);
         Console.WriteLine("Value of Num: {0}", getNum());
         Console.ReadKey();
      }
   }
}
When the above code is compiled and executed, it produces the following result −
Value of Num: 75


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