What are "Delegates" in C#

Delegate in C#

What is Delegate?
Delegate is type which holds the method(s) reference in an object. It is also reffered as a type safe function pointers.

Advantages:

Encapsulating the method's call from caller.
Effective use of Delegat improves the performance of application.
Used to call a method asynchronously.

Declaration of Delegate

public delegate type_of_delegate delegate_name()

Example : public delegate int testdelegate(int delegate variable1,int delegate variable2)

Importan point in Delegate

you can use delegeate without parameter or with parameter list.
you should follow the same syntax as in the method.

(if you are reffering the method with two int parameters and int return type the delegate which you are declaring should be the same format.This is how it is reffered as type safe function pointer)

Declaring and Implementing a Delegate: SimpleDelegate.cs

using System; (namespace)

// this is the simple delegate declaration
public delegate int Comparer(object obj1, object obj2);

public class Name
{
public string FirstName = null;//
public string LastName = null;

public Name(string first, string last)
{
FirstName = first;
LastName = last;
}

// this is the delegate method handler
public static int CompareFirstNames(object name1, object name2)
{
string n1 = ((Name)name1).FirstName;
string n2 = ((Name)name2).FirstName;

if (String.Compare(n1, n2) > 0)
{
return 1;
}
else if (String.Compare(n1, n2) < 0)
{
return -1;
}
else
{
return 0;
}
}

public override string ToString()
{
return FirstName + " " + LastName;
}
}

class SimpleDelegate
{
Name[] names = new Name[5];

public SimpleDelegate()
{
names[0] = new Name("Joe", "Mayo");
names[1] = new Name("John", "Hancock");
names[2] = new Name("Jane", "Doe");
names[3] = new Name("John", "Doe");
names[4] = new Name("Jack", "Smith");
}

static void Main(string[] args)
{
SimpleDelegate sd = new SimpleDelegate();

// this is the delegate instantiation
Comparer cmp = new Comparer(Name.CompareFirstNames);

Console.WriteLine("\nBefore Sort: \n");

sd.PrintNames();

// observe the delegate argument
sd.Sort(cmp);

Console.WriteLine("\nAfter Sort: \n");

sd.PrintNames();
}

// observe the delegate parameter
public void Sort(Comparer compare)
{
object temp;

for (int i=0; i < names.Length; i++)
{
for (int j=i; j < names.Length; j++)
{
// using delegate "compare" just like
// a normal method
if ( compare(names, names[j]) > 0 )
{
temp = names;
names = names[j];
names[j] = (Name)temp;
}
}
}
}

public void PrintNames()
{
Console.WriteLine("Names: \n");

foreach (Name name in names)
{
Console.WriteLine(name.ToString());
}
}
}


Multicast Delegate :

Delegates which hold and invoke multiple methods are known as multicast delegates. But simple delegates invoke only one method. Multicast delegates must satisfy the following condition:

· The return type of the delegate must void.

· None of the parameters of the delegate type can be declared as output parameters, using out keywords.


For Example:

delegate void Multidelegate();

class Program

{

public static void Main()

{

Multidelegate mshow1 = new Multidelegate(Class1.show1);

Multidelegate mshow2 = new Multidelegate(Class1.show2);

Multidelegate mshow3 = mshow1 - mshow2;

Multidelegate mshow4 = mshow2 - mshow1;

Multidelegate mshow5 = mshow3 - mshow2;

Multidelegate mshow6= mshow5 - mshow4;

mshow3();

mshow4();

mshow5();

mshow6();

Console.ReadLine();

}

}

class Class1

{

static public void show1()

{

Console.WriteLine("Biswa");

}

static public void show2()

{

Console.WriteLine("Sourav");

}

}

Output:

Biswa

Sourav

Biswa

Biswa

Thanks for the article. What does it mean type safe function pointer ?

Read more about C# Delegates on this URL : http://planetofcoders.com/c-delegates/

Hi all,

Can u tell me

Why delegate are used in c#
What is ths use of delegate
can u give real time example.