genericsTest.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Mime;
using System.Text;
using System.Threading.Tasks;

namespace StackOverflow
{
    public interface A { };
    public interface B : A { };
    public interface C : A { };
    public interface D : B { };

    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
    {

    };

    class Program
    {
        public IMyInterface<T> Create<T>() where T : A, B, C
        {

            if (typeof(T) is B)
            {
                return new FirstImpl<T>();
            }
            if (typeof(T) is C)
            {
                return new SecondImpl<T>();
            }
            if(typeof(T) is D)
            {
                return new FirstImpl<T>();
            }
            return null;
        }

        static void Main(string[] args)
        {
          
        }
    }
}

to be more specific (and I hope that formatting works in comments)

namespace StackOverflow
{
public interface A { };
public interface B : A { };
public interface C : A { };
public class D : B { };
public class E : C { };

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
{
};

class Program
{
    public IMyInterface<T> Create<T>() where T : A, B, C
    {
        if (typeof(T) is B)
        {
            return new FirstImpl<T>();
        }
        if (typeof(T) is C)
        {
            return new SecondImpl<T>();
        }
        return null;
    }

    static void Main(string[] args)
    {
          var correctImplementation = Create<D>();  // should be used FirstImpl
          var correctImplementation2 = Create<E>();  // should be used SecondImpl
    }
}

}