- C# delegates are similar to pointers to functions, in C or C++.
- A delegate is a reference type variable that holds the reference to a method. The reference can be changed at runtime.
- Delegates are especially used for implementing events and the call-back methods.
- All delegates are implicitly derived from the System.Delegate class.
- Provides a good way to encapsulate the methods.
- Delegates are the library class in System namespace.
- These are the type-safe pointer of any method.
- Delegates are mainly used in implementing the call-back methods and events.
- Delegates can be chained together as two or more methods can be called on a single event.
- It doesn’t care about the class of the object that it references.
- Delegates can also be used in “anonymous methods” invocation.
- Anonymous Methods(C# 2.0) and Lambda expressions(C# 3.0) are compiled to delegate types in certain contexts. Sometimes, these features together are known as anonymous functions.
public delegate int MyDelegate (string s);
delegate <return type> <delegate-name> <parameter list>
Instantiating Delegates
public delegate void printString(string s);
...
printString ps1 = new printString(WriteToScreen);
printString ps2 = new printString(WriteToFile);
delegate int NumberChanger(int n);
namespace DelegateAppl {
class TestDelegate {
static int num = 10;
public static int AddNum(int p) {
num += p;
return num;
}
public static int MultNum(int q) {
num *= q;
return num;
}
public static int getNum() {
return num;
}
static void Main(string[] args) {
//create delegate instances
NumberChanger nc1 = new NumberChanger(AddNum);
NumberChanger nc2 = new NumberChanger(MultNum);
//calling the methods using the delegate objects
nc1(25);
Console.WriteLine("Value of Num: {0}", getNum());
nc2(5);
Console.WriteLine("Value of Num: {0}", getNum());
Console.ReadKey();
}
}
}
Value of Num: 35
Value of Num: 175
x
A delegate is an object which refers to a method or you can say it is a reference type variable that can hold a reference to the methods. Delegates in C# are similar to the function pointer in C/C++. It provides a way which tells which method is to be called when an event is triggered.
Important Points About Delegates:
Delegate declaration determines the methods that can be referenced by the delegate. A delegate can refer to a method, which has the same signature as that of the delegate.
For example, consider a delegate −
The preceding delegate can be used to reference any method that has a single string parameter and returns an int type variable.
Syntax for delegate declaration is −
Once a delegate type is declared, a delegate object must be created with the new keyword and be associated with a particular method. When creating a delegate, the argument passed to the new expression is written similar to a method call, but without the arguments to the method. For example −
Following example demonstrates declaration, instantiation, and use of a delegate that can be used to reference methods that take an integer parameter and returns an integer value.
When the above code is compiled and executed, it produces the following result −
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