Understanding Delegates in C#

Delegates are a mechanism of indirectly calling methods at runtime. The primary use of delegates in C# programming is for implementing events and the call-back methods.

Important Points About Delegate

• Delegate is a strongly typed function pointer (just like pointer is there in C++ language).
• A Delegate is a reference type variable which holds the reference of a method.
• We can call delegate like methods in C#.
• Delegates are used to raise the event in class and associated with the event handler.
• Delegate can be Non-Generic/ Generic.

Types of Delegate

There are two types of delegate.

Note 

Generics are a kind of parameterized data structure that can work with value type as well as reference type. Using generics we can define the data type at runtime using System.Collections.Generic namespace.

• Single cast Delegate
  
  
  1. Single cast Delegate: A single cast delegate is derived from System.Delagate Class.
  2. It contains reference of only one method at a time.
     
     
• Multi-cast Delegate
  
  
  1. Multi-cast Delegate: Multicast Delegate is derived from System.MulticatsDelagate class.
  2. It Creates an invocation list of multiple methods.
  3. In Multicast delegate we create a single delegate that invokes multiple methods.
  4. It holds the reference of multiple methods and it executes all the methods as in calling order.
  5. The multiple method called by delegate should not return any values.

Syntax

< public delegate return Type(void/string) delegate_Name (); >

Or

< public delegate return Type(void/string) delegate_Name <T> (); >

Example

Single Cast Non-Generic/Generic Delegate,

namespace Demo_Delegate
{

    class Single_Cast
    {

        //Declare Non-Generic Single cast delegate
        public delegate void Single_Cast_delegate();

        //Private Method to be called
        private  static void message()
        {
            Console.WriteLine("This Example of Single Cast Non-generic Delegate");
        }
        static void Main(string[] args)
        {
            //Instance created for Single Cast Delagate
            Single_Cast_delegate SCD = new Single_Cast_delegate(message);

            //Instance created for Single Cast Generic Delagate
            Single_Cast_Generic SCG = new Single_Cast_Generic();
            Mydel<int> my1 = new Mydel<int>(SCG.add);
            Mydel<string> my2 = new Mydel<string>(SCG.Myname);

         //Calling of Delegate
            SCD();
            my1(10, 20);
            my2("My", "Name");
        }

    }

    //Declare Generic Single cast delegate
    public delegate void Mydel<T>(T a, T b);
    class Single_Cast_Generic
    {
        public void add(int a, int b)
        {
            int Sum = a + b;
            Console.WriteLine(Sum);

        }

        public void Myname(string firstname, string lastname)
        {
            string fn = firstname + " " + lastname;
            Console.WriteLine(fn);

        }
    }
}

Multicast Non-Generic/Generic Delegate,

namespace Demo_Delegate
{
    class MyClass
    {
        static void Main(string[] args)
        {
            //Instance created for Muticaste Cast Delagate
            MultiCast_delegate md = new MultiCast_delegate();
            NonGenericMulticaste NGM = new NonGenericMulticaste(md.FirstMessage);

            // Add another method to the invocation list
            NGM += new NonGenericMulticaste(md.SecondMessage);
            NGM("Call Mutliple methods using single Object");


            //Instance created for MultiCast Generic Delagate
            MultiCast_Genericdelegate MG = new MultiCast_Genericdelegate();

            //We can call both method having diffrent type with single delegate
            GenericMulticaste<int> obj1 = new GenericMulticaste<int>(MG.FirstMessage);
            obj1 += new GenericMulticaste<int>(MG.Message);

            GenericMulticaste<string> obj2 = new GenericMulticaste<string>(MG.SecondMessage);
            obj2 += new GenericMulticaste<string>(MG.SecondMessage);

            obj1(100);

            obj2("Call Mutliple Generic methods using single Object");



        }
    }
    //Create a multicate Delegate
    public delegate void NonGenericMulticaste(string Message);
    class MultiCast_delegate
    {
        public void FirstMessage(string Message1)
        {
            Console.WriteLine(Message1);
        }
        public void SecondMessage(string Message2)
        {
            Console.WriteLine(Message2);
        }
    }
    //Create a multicate Generic Delegate
    public delegate void GenericMulticaste<T>(T Message);
    class MultiCast_Genericdelegate
    {
        public void FirstMessage(int Number)
        {
            Console.WriteLine("Your Number is:"+Number);
        }
        public void Message(int Number)
        {
            Console.WriteLine("Your Number is:" + Number);
        }
        public void SecondMessage(string Generic_Message)
        {
            Console.WriteLine(Generic_Message);
        }
        public void Second(string Generic_Message)
        {
            Console.WriteLine(Generic_Message);
        }
    }
}

Following are the predefined generic delegates,

public delegate TResult Func<in T, out TResult>(T arg);
public delegate void Action()
public delegate bool Predicate<in T>(T obj);

Example

namespace PreDefinedGenericDelegates
{
    class Program
    {
        static void Main(string[] args)
        {
            //Action Delegate
            Action<int,int> a = new Action<int,int>(MyClass.sum);
            a(5, 6);
            Action<string, string> b = new Action<string, string>(MyClass.add);
            b("Kundan", "Jha");

            //Func Delegate

            Func<int, int, int> ds = new Func<int, int, int>(MyClass.Multi);
            Console.WriteLine(ds(45, 65));

            //Predicate Delegate
            Predicate<string> p = new Predicate<string>(MyClass.IsDate);
            if (p("01-01-2015"))
            {
                Console.WriteLine("Valid");
            }
            else
            {
                Console.WriteLine("Invalid");
            }

            Console.ReadKey();

        }
    }

    class MyClass
    {
        public static bool IsDate(string date)
        {
            DateTime dt;
            return DateTime.TryParse(date, out dt);
        }

        public static int Multi(int a, int b)
        {
            return a + b;
        }
        public static void sum(int num1, int num2)
        {
            int sum = num1 + num2;
            Console.WriteLine(sum);

        }
        public static void add(string fname, string lname)
        {
            string fullname = fname + " " + lname;
            Console.WriteLine(fullname);
        }
    }
}

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