Order of implicit conversions in c#

问题

What is the order of implicit conversions done in Console.WriteLine(x) when x is an object from user-defined class:

    class Vector
    {
        public int x = 12;       

        public static implicit operator double(Vector v1)
        {
            return 3.14;
        }

        public static implicit operator int(Vector v1)
        {
            return 42;
        }

        public override string ToString()
        {
            return this.x.ToString();
        }

    }

    static void Main(string[] args)
    {
        Vector v11 = new Vector();
        Console.WriteLine(v11);
    }

Why I get 42, and not 3.14 or "12"? Why I can not add an additional implicit conversion to string /there is Compiler error on the ambiguity between CW(int) and CW(string)/:

        public static implicit operator string(Vector v1)
        {
            return "42";
        }

I know I should avoid using the implicit cast, but just for curiosity!

回答1:

So what is printed is based entirely on what overload of Console.WriteLine is chosen. Which overload is chosen is based on section 7.5.3.2 of the specs on "betterness" for function members.

An overload is "better" than another, when it has a parameter that is "more specific" than another one. "more specific" means there's an implicit conversion from the more specific type to the less specific type, and no implicit conversion from the less specific type to the more specific type.

object is the least specific overload, as there's no implicit conversion from it to int, double, or string, but there is one from every type to it. int is more specific than double because there's an implicit conversion from int to double, but no conversion from double to int. int and string have no implicit conversions between each other, so neither is more specific, and so neither is better or worse than the other.

So if there's an implicit conversion from your object to string then the string overload is considered, and there's a tie for "best" overload, and you get an error. When it's missing there's a "most specific" type of all of the considered overloads (which is int), so it's "the best", and that overload is chosen.

来源：https://stackoverflow.com/questions/53615219/order-of-implicit-conversions-in-c-sharp