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what is a delegate?
a delegate is a type that defines a method signature, and can be associated with any method with a compatible signature. (MSDN)

Declaring a delegate:
public delegate int PerformCalculation(int x, int y);
or
public delegate void TestDelegate(string message);

Demo:
using System;
public delegate void TestDelegate(string message); //Declare the delegate
class Test
{
  public static void Display(string message)
  {
    Console.WriteLine("The string entered is : " + message);
  }
  static void Main()
  {
    TestDelegate t = new TestDelegate(Display); //Instantiate the delegate
    Console.WriteLine("Please enter a string");
    string message = Console.ReadLine();
    t(message); //Invoke the delegate
    Console.ReadLine();
  }
}