C＃无法重载+如果第二个参数是枚举

Question

I have the following signature for overloaded + :

    public static double operator +(MyClass x, MyEnum e)

and an expression of the form:

    x.Value = someMyClassValue + MyEnum.X;

The behavior the debugger shows is as if the expression had been:

    x.Value = MyEnum.X;

The overload never gets called.

I also have:

    public static double operator +(MyClass x, object o)

but that doesn't get called either for enums, though it does for other cases.

I also have overloads for string, int, float, double, and they all work perfectly. Why is enum a special case, and why the odd behavior? Could this be a bug in the Mono compiler?

I'm using Mono 2.10.8.1 on Ubuntu 13.04.

Afternote

The problem was that I had also defined an implicit cast to int . See my answer for details.

Answer 1

The problem was that I had also defined:

public static implicit operator int(MyClass o)

The implicit cast takes precedence over the overloaded operator, and the whole addition expression takes the type of the enum.

Since I wanted to keep the implicit cast to int , I adopted this solution:

    public enum MyEnum : ulong

With that, the cast to int no longer takes place.

The following program demonstrates the problem I was having. It's output is "SECOND" instead of "OK".

using System;

public class EnumPlus
{
    public enum Constant
    {
        FIRST,
        SECOND
    };

    // if this implicit cast is removed the result is what I expected
    public static implicit operator int(EnumPlus f)
    {
        return 1;
    }

    public static string operator+(EnumPlus o, int i)
    {
        Console.WriteLine("operator + called for int");
        return "BAD";
    }

    public static string operator+(EnumPlus o, Constant Constant)
    {
        Console.WriteLine("operator + called for enum");
        return "OK";
    }

    public static void Main()
    {
        EnumPlus o = new EnumPlus();
        Console.WriteLine(o + Constant.FIRST);
    }

}