A delegate in C# is similar to a function pointer in C or C++. Using a delegate allows the programmer to encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code which can call the referenced method, without having to know at compile time which method will be invoked.
Declaration
public delegate type_of_delegate delegate_name()
Example:
public delegate int mydelegate(int delvar1,int delvar2)
Note
· You can use delegates without parameters or with parameter list
· You should follow the same syntax as in the method
(If you are referring to the method with two int parameters and int return type, the delegate which you are declaring should be in the same format. This is why it is referred to as type safe function pointer.)
(If you are referring to the method with two int parameters and int return type, the delegate which you are declaring should be in the same format. This is why it is referred to as type safe function pointer.)
Sample Program using Delegate
public delegate double Delegate_Prod(int a,int b);
class Class1
{
static double fn_Prodvalues(int val1,int val2)
{
return val1*val2;
}
static void Main(string[] args)
{
//Creating the Delegate Instance
Delegate_Prod delObj = new Delegate_Prod(fn_Prodvalues);
Console.Write("Please Enter Values");
int v1 = Int32.Parse(Console.ReadLine());
int v2 = Int32.Parse(Console.ReadLine());
//use a delegate for processing
double res = delObj(v1,v2);
Console.WriteLine ("Result :"+res);
Console.ReadLine();
}
}
Explanation
Here I have used a small program which demonstrates the use of delegate.
The delegate "Delegate_Prod" is declared with double return type and accepts only two integer parameters.
Inside the class, the method named fn_Prodvalues is defined with double return type and two integer parameters. (The delegate and method have the same signature and parameter type.)
Inside the Main method, the delegate instance is created and the function name is passed to the delegate instance as follows:
Delegate_Prod delObj = new Delegate_Prod(fn_Prodvalues);
After this, we are accepting the two values from the user and passing those values to the delegate as we do using method:
delObj(v1,v2);
Here delegate object encapsulates the method functionalities and returns the result as we specified in the method.
Multicast Delegate
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.
Simple Program using Multicast Delegate
delegate void Delegate_Multicast(int x, int y);
Class Class2
{
static void Method1(int x, int y)
{
Console.WriteLine("You r in Method 1");
}
static void Method2(int x, int y)
{
Console.WriteLine("You r in Method 2");
}
public static void Main()
{
Delegate_Multicast func = new Delegate_Multicast(Method1);
func += new Delegate_Multicast(Method2);
func(1,2); // Method1 and Method2 are called
func -= new Delegate_Multicast(Method1);
func(2,3); // Only Method2 is called
}
}
Explanation
In the above example, you can see that two methods are defined named method1 and method2 which take two integer parameters and return type as void.
In the main method, the Delegate object is created using the following statement:
Delegate_Multicast func = new Delegate_Multicast(Method1);
Then the Delegate is added using the += operator and removed using the -= operator.
l An eventing design pattern is used.
l It is desirable to encapsulate a static method.
l The caller has no need to access other properties, methods, or interfaces on the object implementing the method.
l Easy composition is desired.
l A class may need more than one implementation of the method.
l We can use delegates whenever we need to pass methods as parameters to another functions so that they an be executed at runtime from there
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