using System;

using System.Threading.Tasks;

using System.Collections.Concurrent;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var widgetBag = new ConcurrentBag<Widget>();

​

        Parallel.For(0, 100000,  (i) => {

            var w = new Widget();

            widgetBag.Add(w);

        });

        //Check the results for duplicates:

        var dupes = widgetBag.GroupBy(w => w.ID)

                             .Where(g => g.Count() > 1)

                             .Select(g => g.Key);

        if (dupes.Any()) 

            Console.WriteLine("Bad news - duplicates exist, count is: " + dupes.Count());

    }

}

​

public class Widget {

    private static int _NextId;

    private readonly object _LockToken = new object();

​

    public Widget() {

        //I expected this lock to prevent any other thread from assigning its ID until we have

        //completed incrementing the member variable

        lock (_LockToken) {

            ID = ++_NextId;

        }

    }

​

    public int ID { get; private set; }

}