using System.Collections.Generic;

​

public class A {}

public class B: A {}

public interface IP<out T> where T: A {

}

public class P<T>: IP<T>where T: A {

    T _a;

    public P(T a) => _a = a;

}

public class Program

{

    public static void Main()

    {

        var l = new List<IP<A>>();

        var p = new P<B>(new B());

        l.Add(p);

    }

}