using System;

using System.Collections.Generic;

using System.Linq;

​

public class Reviewers {

    public string UserId {get;set;}

}

​

public class Review {

    public List<Reviewers> Reviewers { get; set; } = new List<Reviewers>();

}

public class Program

{

    public static void Main()

    {

        var Review = new Review {

            Reviewers = new List<Reviewers>() {

                new Reviewers { UserId = "1" },

                new Reviewers { UserId = "2" }

            }

        };

        List<string> newReviewers = new List<string> { "1", "20", "500" };

        // -- Linq operations --

        var inList = Review.Reviewers.Where(p => !newReviewers.Contains(p.UserId));

        var newList = newReviewers.Aggregate(new List<Reviewers>(), (accu, next) => {

            if (inList.Where(j => j.UserId == next).Count() > 0) {

                return accu;

            }

            return accu.Concat(new List<Reviewers>() { new Reviewers { UserId = next } }).ToList();

        });

        // --------

        Console.WriteLine("OLD:");

        Review.Reviewers.ForEach(p => Console.WriteLine(p.UserId));

        Console.WriteLine("");

        Console.WriteLine("New:");

        newList.ForEach(p => Console.WriteLine(p.UserId));

    }

}