可以将C＃扩展方法添加到F＃类型中吗？

Question

I've got somebody's F# library with a type in it:

module HisModule

type hisType {
    a : float;
    b : float;
    c : float;
}

I'm using it in C#, and I would like to add a "ToString()" method to it, in order to facilitate debugging.

But the following doesn't seem to work:

public static class MyExtensions
{
    public static string ToString(this HisModule.hisType h)
    {
        return String.Format("a={0},b={1},c={2}", h.a, h.b, h.c);
    }
}
....
var h = new hisType();
Console.WriteLine(h.ToString());  // prints "HisModule+hisType"

Any ideas why not?

Answer 1

As others have pointed out, the ToString on object will always be a better match than your extension method. You should probably change the signature of your extension method; changing the name is probably the right way to go.

Moreover: you said that the purpose of this thing was to facilitate debugging. Overriding ToString might be the wrong thing to do there; ToString might be used for something other than debugging. I would be inclined to make my own specially-named method whose name clearly reflects the purpose of the method.

If you are creating a new type and want to have special display behaviour in the debugger, the easiest thing to do is to use the Debugger Display Attributes .

If you want to get really fancy to display a complex data structure in an interesting way, consider writing a Debugger Visualizer .

Answer 2

The answer to your question is "yes". Your sample does not succeed, however, because method resolution succeeds when it finds object.ToString() , so the compiler never looks for extension methods. Try it with a different name:

public static class MyExtensions 
{ 
    public static string Foo(this HisModule.hisType h) 
    { 
        return String.Format("a={0},b={1},c={2}", h.a, h.b, h.c); 
    } 
} 

....   
var h = new hisType();   
Console.WriteLine(h.Foo());

Answer 3

I think you can not do that, as ToString() is always there, in any object of CLR world. Check out Eric Lippert answer .

Answer 4

You could create a wrapper type (with an implicit conversion) that overrides ToString .

class MyType {
    private readonly hisType _hisType;
    private MyType(hisType hisType) {
        _hisType = hisType;
    }
    public static implicit operator MyType(hisType hisType) {
        return new MyType(hisType);
    }
    public override string ToString() {
        return String.Format("a={0},b={1},c={2}", _hisType.a, _hisType.b, _hisType.c);
    }
}

hisType y;
MyType x = y;