Why can't a delegate refer to a non-static method when used in a static method?

问题

Why is it necessary to make a function STATIC while using delegates in C# ?

class Program
{
    delegate int Fun (int a, int b);
    static void Main(string[] args)
    {
        Fun F1 = new Fun(Add);
        int Res= F1(2,3);
        Console.WriteLine(Res);
    }

   **static public int Add(int a, int b)** 
    {
        int result;
        result = a + b;
        return result;
    }
}

回答1:

It's not "necessary". But your Main method is static, so it can't call a non-static method. Try something like this (this isn't really a good way to do things—you really should create a new class, but it doesn't change your sample much):

class Program 
{ 
    delegate int Fun (int a, int b); 
    void Execute()
    {
       Fun F1 = new Fun(Add); 
       int Res= F1(2,3); 
       Console.WriteLine(Res); 
    }

    static void Main(string[] args) 
    { 
        var program = new Program();
        program.Execute();
    } 

    int Add(int a, int b)
    { 
        int result; 
        result = a + b; 
        return result; 
    } 
}

回答2:

Your function needs to be static because you're calling from a static method, Main. You can make the method non-static:

class Program
{
    delegate int Fun (int a, int b);
    static void Main(string[] args)
    {
        Program p = new Program();       // create instance of Program
        Fun F1 = new Fun(p.Add);         // now your non-static method can be referenced
        int Res= F1(2,3);
        Console.WriteLine(Res);
    }

    public int Add(int a, int b)
    {
        int result;
        result = a + b;
        return result;
    }
}

回答3:

In this case, because you aren't creating an instance of any class, the only alternative is a static function. Were you to instantiate an object of type Program, then you could use an instance method instead.

回答4:

Delegates basically follow the same rules as methods. In the example provided your delegate must be static because you are calling it from a static method. In the same vein this will not work:

static void Main(string[] args)
{
    int Res = Add(3, 4);
    Console.WriteLine(Res);
}

public int Add(int a, int b)
{
    int result;
    result = a + b;
    return result;
}

However if you moved things into a non static context like this:

class MyClass
{
    public MyClass()
    {
        Fun F1 = new Fun(Add);
        int Res = F1(2, 3);
        Console.WriteLine(Res);
    }

    public int Add(int a, int b)
    {
        int result;
        result = a + b;
        return result;
    }
}

You can have a delegate with a non-static method.