using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        // First List

        List<int> userIds = new List<int>

        {

            1,

            3,

            5,

            9

        };

        // Seconds List

        List<Fubar> widgets = new List<Fubar>();

        widgets.Add(new Fubar { Id = 1, Name = "Tom" });

        widgets.Add(new Fubar { Id = 2, Name = "Dick" });

        widgets.Add(new Fubar { Id = 3, Name = "Harry" });

        // Get Ids so we can do compare

        List<int> widgetIds = widgets.Select(x => x.Id).ToList();

    //  Console.WriteLine(string.Join(";", widgetIds));

     Console.WriteLine("compare widgets to userIds" );

        Console.WriteLine("");

        Console.WriteLine("");

        Console.WriteLine("Get matches");

        List<int> matches =  widgetIds.Intersect(userIds).ToList();

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

        Console.WriteLine("");

        Console.WriteLine("");

     Console.WriteLine("Do we have any entries in widgets that do not exist in userIds ?" );

     Console.WriteLine("if yes then add them" );

        List<int> widgetsNotInUserIds  = widgetIds.Except(userIds).ToList();

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

        Console.WriteLine("");

        Console.WriteLine("");

        Console.WriteLine("Do we have any entries in userIds that do not exist in widgets?" );

        Console.WriteLine("if yes then delete them");

        List<int> userIdsNotInWidgets = userIds.Except(widgetIds).ToList();

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

    }

​

    public class Fubar

    {

        public int Id { get; set; }

        public string Name { get; set; }

    }

}