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ABC’s of Delegate

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6 Jan 2011CPOL
This article helps you to understand the basics of delegate in .NET Framework.

This article helps you to understand the basics of delegate in .NET Framework.

What is Delegate

During C/C++ time, function pointer was a buzzword using which, you can point to a function and call the pointer in order to invoke the function. In .NET, there is nothing called pointer so .NET comes up with delegate. In one sentence, delegate is a glorified pointer to a function. You can call a static or instance method (function) using traditional classname.methodname or just pointing the method to delegate and call the delegate which, in turn, will call the method.

Delegate provides a mechanism to defining and executing callback. This allows you to define the exact signature of callback and that defines the type of delegate. In general, delegate instance consists of fields like a reference to an object and a pointer to an instance method. However, if the object reference is null, CLR understands it as a static method.

When you declare a delegate, C# compiler generates a class derives from MulticastDelegate. MulticastDelegate contains several methods but you cannot see those methods if you view the IL of your code in ILDASM. It is because CLR implements all these methods dynamically at runtime.

Delegate allows classes not to coupled strongly. Also callerof the delegate has no idea whether it is calling a static method or instance method or what exact method it is calling. Think about the GUI Button class. Button class doesn’t know which method it will execute onClick event.

Just think, if we write the code within the button class OnClick event, then would it be reusable class? Button class publishes the OnClick event using a delegate and informs that you can subscribe the OnClick event with a method. In this way, you can de-couple your classes using delegate.

How to call Delegate

Before delving into code, you need to understand one very simple rule about delegate. The return-type or parameter list of the method that you want to reference by delegate, should match with delegate type declaration.Confused! Take an example

Suppose I have one method like void PrintToConsole(string message). The delegate type should be delegate void objDelegate(string message). Now if you write objDelegate =PrintToConsole then it is perfect. Call the function using objDelegate(“Hello World”)which will execute the PrintToConsole method.

So what does following statement mean?

public delegate void _delagate(int i,int j);

It means a declaration of delegate that can encapsulate any method which will take two integer parameters and return void.

When you declare above line, C# compiler produces following code snippet

public class NotifyGpaDelegate : System.MulticastDelegate
{
public void Invoke(int GPAScore)
{
// code
}
} 

This Invoke method is part of Multicast Delegate and should have the same signature of the defined delegate.

Simple Delegate

public delegate void _MathDelegate(int x,int y);
public class clsMath
{
public void Sum(int i, int j)
{
Console.WriteLine(i + j);
}
public static void Subtract(int i, int j)
{
Console.WriteLine(i – j);
} 
}
class Program
{ 
static void Main(string[] args)
{
clsMath _math = newclsMath();
_MathDelegate _delegate1,_delegate2;
_delegate1 = _math.Sum;
_delegate2 = clsMath.Subtract;
_delegate1(100, 50);
_delegate2(100, 50);
}
}

In the above code, I defined a delegate which return void and take two integer parameters. Since, I want to call the Sum/Subtract method using this delegate, you can see I have made them signature compatible between delegate and Sum/Subtract method. In my main method, I defined delegate, reference delegate with the method and callthe delegates, which in turn call the methods. This is an example of simple delegate.

Delegate Chaining

It means you can maintain a list of delegates which can be called. Internally it maintains a linked list of delegates and when the head of the list is called, all the delegates in the chain are called. In Delegate class, following methods help toform a delegate chain.

public abstract class Delegate : ICloneable,ISerializable
{
public static Delegate Combine(paramsDelegate[] delegates);
public static Delegate Combine(Delegatea, Delegate b);
}

The first method takes anarray of delegates and return delegate type. You can also remove the delegate from delegate chain using following methods defines in Delegate class

public abstract class Delegate : ICloneable, ISerializable
{
public static Delegate Remove(Delegatesource, Delegate value);
public static Delegate RemoveAll(Delegatesource, Delegate value);
}

The source is the delegate chain and value is the delegate that you want to remove from delegate chain. In order to make programming simple, C# compiler uses operator overloading to implement these methods. You can use + to combine and – to remove from delegate chain.

public delegate void _MathDelegate(int x,int y);
public class clsMath
{
public void Sum(int i, int j)
{
Console.WriteLine(i + j);
}
public static void Subtract(int i, int j)
{
Console.WriteLine(i – j);
} 
}
class Program
{ 
static void Main(string[] args)
{
clsMath _math = newclsMath();
_MathDelegate[] _delegate = new _MathDelegate[]{_math.Sum,clsMath.Subtract};
_MathDelegate _chainDelegate = _delegate[0] +_delegate[1];
_chainDelegate(100, 50);
}
}

In the above example, I define one array of delegates which contains all instance and static methods. You can combine those methods using  +operator which will call Sum and Subtract method respectively.

_MathDelegate _chainDelegate = _delegate[0] +_delegate[1];
_MathDelegate _chainDelegate1 = _chainDelegate -_delegate[0];

If I use above two statements, CLR interprets it as (Sum + Subtract) – Sum and execute only Subtract method. This is an example of removing delegate from delegate chain using – operator.

You can also iterate through the delegate chain.

public delegate int _MathDelegate(int x,int y);
public class clsMath
{
public int Sum(int i, int j)
{
return (i + j);
}
public static int Subtract(int i, int j)
{
return (i – j);
} 
}
class Program
{ 
static void Main(string[] args)
{
int iTemp = 0;
clsMath _math = newclsMath();
_MathDelegate[] _delegate = new _MathDelegate[]{_math.Sum,clsMath.Subtract};
_MathDelegate _chainDelegate = _delegate[0] +_delegate[1];
Delegate[] _delegates =_chainDelegate.GetInvocationList();
for (int i = 0; i< _delegates.Length; i++)
{
_MathDelegate_del = (_MathDelegate)_delegates[i];
iTemp += _del(100, 50);
}
Console.WriteLine(iTemp);
}
}

In the above example, I used GetInvocationList method to get all the delegates in the delegate chainand call each delegate and summed it up. This method helps us to reference each delegate in the delegate chain and we can also call delegate from the delegate chain in any order.

What is MulticastDelegate

A delegate can call more than one method when invoked. This is referred to as multicasting. A useful property of delegate objects is thatthey can be assigned to one delegate instance to be multicast 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. Also multicast delegate return type is always void.

We can change the implementation of the above code because Multicast Delegate return type should be void.

An Example

class Program
{
// Simple Sum method
public static void Sum(int i, int j)
{
Console.WriteLine(i + j);
}
// Simple Subtract method
public static void Subtract(int i, int j)
{
Console.WriteLine(i – j);
}
// Declaring a simple delegate which will
// encapsulate a Sum/Subtract method
public delegate void _delegate(int x, int y);
static void Main(string[] args)
{
// Declaring objects of Delegate
_delegate objDelegate1, objDelegate2,objDelegate3, objDelegate4;
// Referencing Sum by the delegate object
objDelegate1 = Sum;
// Referencing Subtract by the delegate object
objDelegate2 = Subtract;
// This will call Sum and Subtract both
objDelegate3 = objDelegate1 + objDelegate2;
objDelegate3(100, 50);
// This will call only Subtract
objDelegate4 = objDelegate3 – objDelegate2;
objDelegate4(100, 50); 
}
}

objDelegate3 = objDelegate1 + objDelegate2 means it will call Sum and Subtract method both.

objDelegate4 = objDelegate3 – objDelegate2 means (Sum +Subtract) – Subtract, which means that call to Sum then Subtract and then Sum again.

Few basic thingsabout Delegate

  • Delegates are object oriented, type safe andsecure.
  • Delegate types are derived from Delegate class in .NET.
  • Delegate types are sealed so you cannot inherit Delegate.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

De, Subhendu
Technical Lead
India India
A community lover and consultant working with Microsoft suite of products. Since 2005, he has been working in application development and maintenance using different Microsoft products. During his leisure time, he prefers to serve different Microsoft communities and learning new technologies. You can reach him at his blog DotNet Artisan

Comments and Discussions

 
QuestionDiff b/w composed delegates and multi-cast delegates. Pin
bpc198928-Jan-16 3:41
Memberbpc198928-Jan-16 3:41 
GeneralMy vote of 3 Pin
msadeqian24-Apr-11 22:39
Membermsadeqian24-Apr-11 22:39 
GeneralMy vote of 1 Pin
SledgeHammer016-Jan-11 11:13
MemberSledgeHammer016-Jan-11 11:13 

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Technical Blog
Posted 6 Jan 2011

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