What is Delegate?

I am confused that what is the actual role of a delegate?

I have been asked this question many times in my interviews, but I don't think that interviewers were satisfied with my answer.

Can anyone tell me the best definition, in one sentence, with a practical example?

I like to think of a delegate as "a pointer to a function". This goes back to C days, but the idea still holds.

The idea is that you need to be able to invoke a piece of code, but that piece of code you're going to invoke isn't known until runtime. So you use a "delegate" for that purpose. Delegates come in handy for things like event handlers, and such, where you do different things based on different events, for example.

Here's a reference for C# you can look at:

In C#, for example, let's say we had a calculation we wanted to do and we wanted to use a different calculation method which we don't know until runtime. So we might have a couple calculation methods like this:

public static double CalcTotalMethod1(double amt)
{
    return amt * .014;
}

public static double CalcTotalMethod2(double amt)
{
    return amt * .056 + 42.43;
}

We could declare a delegate signature like this:

public delegate double calcTotalDelegate(double amt);

And then we could declare a method which takes the delegate as a parameter like this:

public static double CalcMyTotal(double amt, calcTotalDelegate calcTotal)
{
    return calcTotal(amt);
}

And we could call the CalcMyTotal method passing in the delegate method we wanted to use.

double tot1 = CalcMyTotal(100.34, CalcTotalMethod1);
double tot2 = CalcMyTotal(100.34, CalcTotalMethod2);
Console.WriteLine(tot1);
Console.WriteLine(tot2);

a delegate is simply a function pointer.
simply put you assign the method you wish to run your delegate. then later in code you can call that method via Invoke.

some code to demonstrate (wrote this from memory so syntax may be off)

delegate void delMyDelegate(object o);

private void MethodToExecute1(object o)
{
	// do something with object
}

private void MethodToExecute2(object o)
{
	// do something else with object
}

private void DoSomethingToList(delMyDelegate methodToRun)
{
	foreach(object o in myList)
		methodToRun.Invoke(o);
}

public void ApplyMethodsToList()
{
	DoSomethingToList(MethodToExecute1);
	DoSomethingToList(MethodToExecute2);
}

Taken from here

>Q What are delegates?
A When an object receives a request, the object can either handle the request itself or pass the request on to a second object to do the work. If the object decides to pass the request on, you say that the object has forwarded responsibility for handling the request to the second object.

Or, as an easy pseudo example: something sends a request to object1. object1 then forwards the request and itself to object2 -- the delegate. object2 processes the request and does some work. (note: link above gives good examples)

Think about delegate as about a simplified implementation of Command pattern.

A delegate is an object that can refer to a method. Thus, when we create a delegate, we are creating an object that can hold a reference to a method. Furthermore, the method can be called through this reference. Thus, a delegate can invoke the method to which it refers. The principal advantage of a delegate is that it allows us to specify a call to a method, but the method actually invoked is determined at runtime, not at compile time.

Simple Delegate

Declaration of delegate:
delegate-modifier delegate return-type delegate-name(parameters)
Implementation of delegate:
Delegate-name delegate-object=new Delegate-name(method of class)

http://knowpacific.wordpress.com/2012/01/26/delegate/

Here I am going to explain delegates, multicast delegates and their usage.. Delegate is a type which holds the method(s) reference in an object. It is also referred to as a type safe function pointer. We can say a delegate is a type that defines a method signature.

When you instantiate a delegate, you can associate its instance with any method with a compatible signature. You can invoke (or call) the method through the delegate instance. Delegates are used to pass methods as arguments to other methods. Event handlers are nothing more than methods that are invoked through delegates. Advantages of using delegates are, Encapsulating the method's call from caller Effective use of delegate improves the performance of application Used to call a method asynchronously. There are some properties of delegates are

Delegates are like C++ function pointers but are type safe.
Delegates allow methods to be passed as parameters.
Delegates can be used to define callback methods.
Delegates can be chained together; for example, multiple methods can be called on a single event.
Methods do not have to match the delegate signature exactly.

public delegate type_of_delegate delegate_name() // Declaration

You can use delegates without parameters or with parameter list
If you are referring to the method with some data type then the delegate which you are declaring should be in the same format. This is why it is referred to as type safe function pointer. Here I am giving an example with String.

The following example shows a delegate operation:

    namespace MyDelegate
    {
        class Program
        {
            private delegate void Show(string s);
    
    
            // Create a method for a delegate.
            public static void MyDelegateMethod(string me

ssage)
        {
            System.Console.WriteLine(message);
        }

        static void Main(string[] args)
        {
            Show p = MyDelegateMethod;
            p("My Delegate");
            p.Invoke("My Delegate");
            System.Console.ReadLine();
        }
    }
}

What is Multicast Delegate?

It is a delegate which holds the reference of more than one method. Multicast delegates must contain only methods that return void, else there is a run-time exception.

 delegate void MyMulticastDelegate(int i, string s);
 Class Class2
 {
  static void MyFirstDelegateMethod(int i, string s)
  {
    Console.WriteLine("My First Method");
  }

  static void MySecondDelegateMethod(int i, string s)
  {
    Console.WriteLine("My Second Method");
  }

  static void Main(string[] args)
  {
    MyMulticastDelegate Method= new MyMulticastDelegate(MyFirstDelegateMethod);
    Method+= new MyMulticastDelegate (MySecondDelegateMethod);
    Method(1,"Hi");             // Calling 2 Methodscalled
    Method-= new MyMulticastDelegate (MyFirstDelegateMethod);
    Method(2,"Hi");             //Only 2nd Method calling
  }
}

Here Delegate is added using the += operator and removed using the -= operator.

Delegate types are derived from the Delegate class in the .NET Framework. Delegate types are sealed—they cannot be derived. Because the instantiated delegate is an object, it can be passed as a parameter, or assigned to a property. This allows a method to accept a delegate as a parameter, and call the delegate at some later time. This is known as an asynchronous callback.

A great explanation and practical implementation of the Delegate pattern can be found in the Google Collections Forwarding Classes (also, the Decorator pattern).

In Event communication sender does not know which object will handle the event. Delegate is type which hold the reference of method. Delegate has signature and holds reference to method which matches its signature so Delegate is like type safe function pointer.

button1.Click += new System.EventHandler(button1_Click) System.EventHandler is declared as a delegate here In .net Events work on the concept of Delegate (like Button Click)

Delegate is used when you do not know which code to invoke at run time So at that time Delegate is used to handle Events

http://msdn.microsoft.com/en-us/library/ms173171(v=vs.80).aspx

Delegates is mainly used with events.

The need is:

You do not want to execute a piece of code at the time when you run the program. After running the program you want to execute that piece of code whenever an event occurs.

Example :

1. Console application - code can be executed only at the time you run the program. (Written inside Main method)
2. Windows application (user interface programming ) - code can be executed when clicking the button after running the program.

This is what they say, you do not know which method will invoke at compiling time. you know it only at runtime that is when clicking the button.

Without delegates no user interface programming is possible. Because you are executing code whenever the user makes events that is clicking button , typing in textbox, selecting dropdownlist item and so on....

A delegate object is an object that another object consults when something happens in that object. For instance, your repair man is your delegate if something happens to your car. you go to your repair man and ask him to fix the car for you (although some prefer to repair the car themselves, in which case, they are their own delegate for their car).

A delegate is an object that represents a pointer to a function. However, it is not an ordinary function pointer in that it:

1. Is Object Oriented

2. Is type safe, i.e. it can only point to a method and you cannot read the raw memory address it is pointing to

3. Is strongly typed. It can only point to methods that match its signatures.

4. Can point to more than one method at the same time.

A delegate is something to which a task is being delegated. The primary purpose of delegation is to decouple code and allow for greater flexibility and reuse.

In programming, and specifically object-oriented programming, this means that when a method is called to do some work, it passes the work on to the method of another object that it has a reference to. The reference could point to whatever object we wish, as long as the object conforms to a predefined set of methods. We call it "programming to an interface" (versus programming to a concrete class implementation). An interface is basically a generic template and has no implementation; it simply means a recipe, a set of methods, preconditions and postconditions (rules).

Simple example:

SomeInterface
{
   public void doSomething();
}


SomeImplementation implements SomeInterface
{
   public void doSomething()
   {
      System.err.println("Was it good for you?");
   }

}


SomeCaller
{
   public void doIt(SomeInterface someInterface)
   {
      someInterface.doSomething();
   }
}

Now you see I can use whatever implementation I want at any time without changing the code in SomeCaller because the type that doIt() is passed is not concrete but rather abstract since it's an interface. In the Java world, this is often expressed in the service paradigm where you call out to a service (an object advertising itself as a service via a specific interface) and the service then calls out to delegates to help it do its work. The service's methods are named as coarse-grained tasks (makePayment(), createNewUser(), etc.), while internally it does lots if nitty-gritty work through delegation, with the delegates' types being interfaces instead of the concrete implementations.

SomeService
{
    SomeInterface someImplementation = ... // assign here
    SomeOtherInterface someOtherImplementation = ... // okay, let's add a second

    public void doSomeWork()
    {
         someImplementation.doSomething();
         someOtherImplementation.doSomethingElse();
    }
}

(N.B.: How an implementation gets assigned is beyond the scope of this thread. Lookup inversion of control and dependency injection.)

While not really a "function pointer", a delegate might look like this is a dynamic language like PHP:

$func = 'foo';
$func();




function foo() {
print 'foo';
}

function foo() {
print 'foo';
}

or in JavaScript you could do something like:

var func = function(){ alert('foo!'); }
func();