using System;

​

public interface A {};

public interface B : A {};

public interface C : A {};

​

public interface IMyInterface<in T> where T : A {};

​

public class FirstImpl<T> : IMyInterface<T> where T : B {};

public class SecondImpl<T> : IMyInterface<T> where T : C {};

​

public class Program

{

    public static void Main()

    {

        var test1 = Create<B>();        

        var test2 = Create<C>();

        Console.WriteLine("test1 " + test1.GetType().Name);

        Console.WriteLine("test2 " + test2.GetType().Name);

    }

    public static IMyInterface<T> Create<T>() where T : A 

    {

        if(typeof(B).IsAssignableFrom(typeof(T)))

        {

            var type = typeof(FirstImpl<>);

            var boundType = type.MakeGenericType(typeof(T));

            return (IMyInterface<T>) Activator.CreateInstance(boundType);

        }

        else if(typeof(C).IsAssignableFrom(typeof(T)))

        {

            var type = typeof(SecondImpl<>);

            var boundType = type.MakeGenericType(typeof(T));

            return (IMyInterface<T>) Activator.CreateInstance(boundType);

        }

        throw new ArgumentException("unknown type " + typeof(T).Name);

    }

}