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Quick Review on Delegates

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Delegates

Here in this article I will explain about delegates in brief. Some important points about delegates. This article is meant to only those who already know delegates, it will be a quick review not a detailed one.

Delegates quite simply are special type of object, a delegate just contains the details of a method.
One good way to understanding delegates is by thinking of delegates as something that gives a name to a method signature.

Syntax:
Public delegate void voidOperation(uint x);
Here we have defined a delegate called voidOperation, and we have indicated that each instance of this delegate can hold a reference to a method that takes one unit parameter and returns void. The crucial point to understand about delegate is that they are very type-safe. When we define the delegate, we have to give full details of the signature of the method that it is going to represent.

Some Important Points on delegates are:
1) Just like a callback functions
2) Function Pointers
3) .Net delegates are type safe.
4) We can define a delegate in any of the same places that we would define a class. Either inside another class or outside of any class and in a namespace as top level object.
5) Defining a delegate is defining a new class.
6) Delegates are implemented as classes derived from the class System.MultiCastDelegate, which is derived from the base class, System.Delegate.
7) After we have defined a delegate, we can create an instance of it so that we can use it to store details of a particular method.
8) Class instance is call an object, where as in case of a delegate, delegate instance is called a delegate itself.
9) Delegates in C# always syntactically take a one-parameter constructor, the parameter being the method to which the delegate will refer. This method must match the signature with which we originally define the delegate.
10) One feature of delegates is that are type-safe to the extent that they ensure the signature of the method being called is correct. However, interestingly they do not care what type of object the method is being called against or even weather the method is a static method or an instance method.
11) An instance of a given delegate can refer to any instance or static method on any object of any type, provided that the signature of the method matches the signature of the delegate.
12)Anonymous Methods:
delegate string delegateTest(string val); //Declaration
delegate anonyDel=delegate(string param) //Here is the //anonymous method.
{
param + = “Bharath Reddy VasiReddy”;
return param;
} //No name give to the set of statements
13) We can group methods together into an array so that we can call several methods in a loop.
Delegate double DoubleOP(double x);
DoubleOp [] operations = {new DoubleOp(MethodName1),new DoubleOp(MethodName2)};

Note: In case of delegate arrays, all the array values should have the same method signature.
14) MultiCast Delegates:
It is possible for a delegate to wrap more than one method. Such a delegate is know as a multicast delegate. If a multicast delegate is called, it will successively call each method in order. For this to make sense, the delegate signature must return a void. If the compiler seens a delegate that returns a void, it automatically assumes we mean a multicast delegate.
Multicast delegate is a class derived from System.MultiCastDelegate, which in turn is derived from System.Delegate. System.MultiCastDelegate has additional members to allow chaining of method calls together into a list.

DoubleOp operation1 = new DoubleOp(methodOperations.MultiplyByTwo);
DoubleOp operation2 = new DoubleOp(MathOperations.Square);
DoubleOp operation = operations1 + operation2;

Operations(value);// This will execute both the functions.

Note: If we are using multicast delegates, we should be aware that the order in which methods chained to the same delegate will be called is formally undefined. We should, therefore avoid writing code that relies on such methods being called in any particular order.




Thanks & Regs
Bharath Reddy VasiReddy
Jun 27 '07 #1
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