Implicit casting of generic classes

Can anyone tell me what am I missing here? Here’s a simple class

public class Mock<TType>


    private readonly TType _actual;


    public Mock(TType actual)


        _actual = actual;


    public TType Actual


        get { return _actual; }


    public static implicit operator TType(Mock<TType> item)


        return item.Actual;



That’s a generic class that defines implicit casting operator to it’s generic parameter, no magic here. The code compiles without warning. Methods like this one:


public TType ThisWorks<TType>()


    return new Mock<TType>(default(TType));


work as well.

What’s even more stunning, whereas this works:

Mock<string> mock1 = _mocks.Mock<string>();

string actual1 = mock1;

This doesn’t:

Mock<ISomethingUseful> mock2 = _mocks.Mock<ISomethingUseful>();

ISomethingUseful actual2 = mock2;

producing error message:

Error    1    Cannot implicitly convert type ‘Mocks.Mock<Mocks.ISomethingUseful>’ to ‘Mocks.ISomethingUseful’. An explicit conversion exists (are you missing a cast?)

Am I missing something here? Why does it say ‘explicit conversion exists’ when I created implicit conversion? Why it doesn’t work for the interface when it works for string?


According to chapter 6.4.4 of C# 3.0 Language specification you just can’t have implicit conversion to an interface. This in yet another time when I hit an invisible wall trying to do something with generics…

And if you thought that

public class MyClass<TType>:TType



could be a solution: it isn’t – this won’t compile as well.

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  • Gauthier Segay


    c# ain’t c++ 😉