Cast to two interfaces

[maxkoshevoi]

I have a generic type that I cannot control that implements multiple interfaces (lets call it Source). And I have a wrapper class Target (that I control) that manipulates members of two non-generic interfaces (that Source implements). I need to create a variable from instance of Source class that I can pass to constructor of Target class.

So something like this. It would be great if t is D d returned true, but it doesn't..

public interface A { }
public interface B { }
public interface C<T> { }

public interface D : A, B { }

public class Source<T> : A, B, C<T> { }
public class Target<T> where T : A, B { /*Constructor and usage of A and B members here*/  }

static void Main()
{
    Source<int> s = new();
    Converter(s);
}

static void Converter(object obj)
{
    if (obj is D d) // false
    {
        new Target(d);
    }
    else if (obj is A && obj is B) // true
    {
        new Target(???);
    }
}

PS: Method where this logic is located accepts object (that may be Source instance) and I cannot change that, so I don't know what type argument instance of Source has.

[Joe4evr]

At first it sounds like you want Shapes/Roles (#164/#1711), but in playing around a bit with your sample you really don't need any of that to get it working.

[maxkoshevoi]

Thanks for the answer! I'll keep a look on those features.

One more thing. Method where this logic is located accepts object (that may be Source instance) and I cannot change that, so I don't know what type argument instance of Source has.

The only way I found to achieve that is to cast t to A and to B separately and then pass two resulting variables into new Target(a, b)

[BreyerW]

What you are looking for is called union&intersection types (eg IA&IB or IA|IB) but c# unfortunetly doesnt support them yet

[bbarry]

This compiles:

public interface A { }
public interface B { }
public interface C<T> { }

public interface D : A, B { }

public class Source<T> : A, B, C<T> { }
public class Target<T> where T : A, B { 
    readonly T _value;
    public Target(T value) {
        _value = value;
    }
    public void DoWork() {}
}

public static class TargetCreator {
    public static Target<T> Create<T>(T value) where T : A, B => new Target<T>(value);
}

public class M {
    static void Main()
    {
        Source<int> t = new();
        if (t is D d) // false
        {
            TargetCreator.Create(d).DoWork();
        }
        else if (t is A && t is B) // true
        {
            TargetCreator.Create(t).DoWork();
        }
    }
}

[maxkoshevoi]

Sorry, I didn't clarify one thing (updated description). Method where this logic is located accepts object (that may be Source instance) and I cannot change that, so I don't know what type argument instance of Source has.

[bbarry]

In that case you have to trick the compiler a little:

public class M {
    static void Accept(object t)
    {
        if (t is D d) // false
        {
            TargetCreator.Create(d).DoWork();
        }
        else if (t is A && t is B) // true
        {
            TargetCreator.Create((dynamic)t).DoWork();
        }
    }
}

[BreyerW]

Using dynamic is not good idea since it pulls in dynamic machinery and performance is lower too. Avoid it if you can.

[gulshan]

See #399 Intersection types for interfaces.