using System;

using System.Collections.Generic;

using System.Linq;

​

public class Object1

{

    public Object1(string str) => String = str;

    public string String { get; set; }  

    public override string ToString() => String;

}

​

public class Object2

{

    public Object2(string str) => String = str;

    public string String { get; set; }  

    public override string ToString() => String;

}

​

public class Program

{

    public static void Main()

    {

        var objects1 = new List<Object1>();

        var objects2 = new List<Object2>();

        objects1.Add(new Object1("A"));

        objects1.Add(new Object1("C"));

        objects1.Add(new Object1("D"));

        objects2.Add(new Object2("A"));

        objects2.Add(new Object2("B"));

        objects2.Add(new Object2("C"));

        objects2.Add(new Object2("D"));

        objects2.Add(new Object2("E"));

        objects2.Add(new Object2("F"));

        objects2 = objects2

            .ExceptBy(

                objects1.Select(obj1 => obj1.String), 

                obj2 => obj2.String)

            .ToList();

        Console.WriteLine(string.Join(", ", objects2));

    }

}

​