using System.Collections.Generic;

public class Program

{

    public static List<Animal> Filter(List<Animal> input) => input;

    public static void Main()

    {

        var cat = new Cat();

        var list = new List<Cat>();

        var newList = Filter(list);

        newList.Add(new Animal());

        newList.Add(new BlackCat());

    }

}

​

public class Animal {}

​

public class Cat: Animal {}

​

public class BlackCat: Cat {}