为什么通用接口约束会混淆扩展方法的使用？

Question

This problem is very difficult for me to explain, so I mocked up some dummy code to show the problem I encountered. It has to do with generic constraints on extension methods. Within my extension method I need the generic class (but the example does not show this). Let's dive in:

I have two interfaces:

public interface IOne { }

public interface ITwo { }

Now I create some extension methods for them:

public static class Extensions
{
    public static string ParseValue(this ITwo ob)
    {
        return "Two";
    }

    public static string ParseValue<T>(this T obj) where T : class, IOne
    {
        return "One";
    }
}

Looks fine right? Now I create an implementation of ITwo and it'll get messy:

public class Two: ITwo
{
    public string Test()
    {
        return this.ParseValue();
    }
}

This class doesn't compile. I get te following error:

The type 'GenericProblem.Two' cannot be used as type parameter 'T' in the generic type or method 'GenericProblem.Extensions.ParseValue(T)'. There is no implicit reference conversion from 'GenericProblem.Two' to 'GenericProblem.IOne'.

How can I solve this problem?

Answer 1

For the error, I've answered it here .

How can I solve this problem?

You need to cast this as ITwo type before calling ParseValue , otherwise compiler will not be able to choose the right match.

public class Two : ITwo
{
    public string Test()
    {
        return ((ITwo)this).ParseValue();
    }
}

Answer 2

Seems pretty straight forward to me.

In your extension class:

public static class Extensions
{
    public static string ParseValue(this ITwo ob)
    {
        return "Two";
    }

    public static string ParseValue<T>(this T obj) where T : class, IOne
    {
        return "One";
    }
}

You do have the method, ParseValue<T> and you put a constraint on it, saying that T must be of type IOne

In your implementation of ITwo, you try to call the extension method on the object this . Obvious this is of type ITwo so your extension method will not work anymore.

Just change the signature of your extension method like:

public static string ParseValue<T>(this T obj) where T : class, ITwo

That should do the trick.

Edit:

According to the original questioner, the solution did not compile. Underneath is the full source code how I have it and this does compile:

public interface IOne { }

public interface ITwo { }

public static class Extensions
{
    public static string ParseValue(this ITwo ob)
    {
        return "Two";
    }

    public static string ParseValue<T>(this T obj) where T : class, ITwo
    {
        return "One";
    }
}

public class Two : ITwo
{
    public string Test()
    {
        return this.ParseValue();
    }
}