Delegate and Events

First, the principle of delegation:

　　In essence, a delegate is a data type that describes a class of methods with the same parameter signature and return value. An instantiated delegate can be thought of as a list of methods with the same parameter signature and return value. Any delegate object is an object of some derived class of System.Delegate.

Second, the definition of delegation:

///Define a delegate
delegate int Plus(int x,int y);

//Instantiate the delegate in the method
public void Test()
{ 
　　 //You can New an instance delegate object, you must specify a function
    Plus p= new SayHello(Num); 
　　 //You can also specify a function directly
    p += Num; 
　　 //The lambda expression is also a function in essence, so it can be written like this
    p +=(x,y)=>x+y; 
　　 When an instantiated delegate specifies multiple methods and all have return values, only the return value of the last method can be received when the delegate is executed
    int a= p(1,2);
}
public int Num (int x, int y)
{
    return x + y;
}  

 

Three, c# built-in delegate, Action, Func

      The c# grammar helps us define two default delegate types, Action and Func. Action delegates can have parameters but no return value. Func has return values ​​and can have parameters.

　　

//Instantiate an Action delegate
Action<string> sayHello=x=>{Console.Write(x);};
sayHello("Have you eaten today?");

//Instantiate a Func delegate
Func<int, int, int> plus = (x, y) => { return x + y; };
var num = plus (1, 8);

　　

Fourth, the use of delegation

　　1. Pass it as a parameter in the method

　　

public int Test()
{ 
　　 //Call the method and pass the lambda expression as a parameter

　　 return Operation((x,y)=> { return x + y; },1,8); 
} 

public int Operation(Func<int,int,int> lam,int a,int b) 
{ 
　　 return lam(a,b); 
}