using System;

using System.Collections.Generic;

public class Program

{

    public class MyClass{

        public MyDerivedClass derived {get;set;}

    }

    public class MyDerivedClass{

        public int MyProp {get;set;}

    }

    public static void Main()

    {

        List<MyClass> myClasses = new List<MyClass>();

        for(var i=1;i<=10; i++){

            var myClass = new MyClass();

            if (i%2==0){

                var devClass = new MyDerivedClass{MyProp=i};

                myClass.derived = devClass;

            }

            myClasses.Add(myClass);

        }

        myClasses.ForEach(x=> Console.WriteLine(x.derived?.MyProp.ToString()));

    }

}