OrderedThreadPool - Bug Fix !!!

Hugh pointed out a bug in the OrderedThreadPool.

I think there is a small window for error in the OrderedThreadPool class. Bascially, if an item of work is queued, then a worker thread runs, takes the item off the queue and is about to call wcb(state) - but at that instant is (say) context switched. Then another item gets queued and another worker thread runs and dequeues the item and then again is about to call wcb(state). There is scope here for the two operations to run concurrently or even out of order...

Here is the fixed version of the same.

using System;
using System.Collections.Generic;
using System.Diagnostics;

namespace System.Threading
{
 public class OrderedThreadPool
 {
  private Queue workItemQ = new Queue();
  private bool loopWorkRunning = false;

  public void QueueUserWorkItem(WaitCallback wcbDelegate, object state)
  {
   lock (workItemQ)
   {
    workItemQ.Enqueue(new ThreadPoolTaskInfo(wcbDelegate, state));

    if (workItemQ.Count == 1 && !loopWorkRunning)
    {
     loopWorkRunning = true;
     ThreadPool.QueueUserWorkItem(LoopWork);
    }
   }
  }

  private void LoopWork(object notUsed)
  {
   WaitCallback wcb = null;
   object state = null;

   lock (workItemQ)
   {
    if (workItemQ.Count == 0)
    {
     loopWorkRunning = false;
     return;
    }

    ThreadPoolTaskInfo tptInfo = workItemQ.Dequeue();
    state = tptInfo.State;
    wcb = tptInfo.CallbackDelegate;
    Debug.Assert(wcb != null);
   }

   try
   {
    wcb(state);
   }
   finally
   {
    ThreadPool.QueueUserWorkItem(LoopWork, notUsed);
   }
  }

  public struct ThreadPoolTaskInfo
  {
   public readonly WaitCallback CallbackDelegate;
   public readonly object State;

   public ThreadPoolTaskInfo(WaitCallback wc, object state)
   {
    Debug.Assert(wc != null);
    CallbackDelegate = wc;
    State = state;
   }
  }
 }
}

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