C# delegate argument type

Question

    delegate void Dele(string str);
    delegate void Alli(int num);

    class Program
    {
        static void Main(string[] args)
        {
            Dele dele = Test; // O
            Alli alli = Test; // X          
        }        

        static void Test(object obj) { }
    } 

Alli alli = Test; // X
Why???

Maybe...
str as object ( O )
num as object ( X )
???

(Sorry, I'm not good at English)
(It looks like your post is mostly code; please add some more details.: OK)

Answer 1

This behaviour is specified in the C# language specification.

Here:

Dele dele = Test;

You are doing a method group conversion . One of the requirements for a method group conversion to be allowed is that

The selected method M must be compatible (Delegate compatibility) with the delegate type D, or otherwise, a compile-time error occurs.

Delegate compatibility is specified like this (emphasis mine):

A method or delegate M is compatible with a delegate type D if all of the following are true:

• D and M have the same number of parameters, and each parameter in D has the same ref or out modifiers as the corresponding parameter in M.
• For each value parameter (a parameter with no ref or out modifier), an identity conversion (Identity conversion) or implicit reference conversion (Implicit reference conversions) exists from the parameter type in D to the corresponding parameter type in M.
• For each ref or out parameter, the parameter type in D is the same as the parameter type in M.
• An identity or implicit reference conversion exists from the return type of M to the return type of D.

There is an implicit reference conversion from string to object , because string is a subclass of object , but there isn't an implicit reference conversion from int to object . int is a value type, so the conversion is actually a boxing conversion . Therefore, the method group conversion does not work for Alli and Test .