Question

I’m trying to implement the Parallel.ForEach pattern and track progress, but I’m missing something regarding locking. The following example counts to 1000 when the threadCount = 1, but not when the threadCount > 1. What is the correct way to do this?

class Program
{
   static void Main()
   {
      var progress = new Progress();
      var ids = Enumerable.Range(1, 10000);
      var threadCount = 2;

      Parallel.ForEach(ids, new ParallelOptions { MaxDegreeOfParallelism = threadCount }, id => { progress.CurrentCount++; });

      Console.WriteLine("Threads: {0}, Count: {1}", threadCount, progress.CurrentCount);
      Console.ReadKey();
   }
}

internal class Progress
{
   private Object _lock = new Object();
   private int _currentCount;
   public int CurrentCount
   {
      get
      {
         lock (_lock)
         {
            return _currentCount;
         }
      }
      set
      {
         lock (_lock)
         {
            _currentCount = value;
         }
      }
   }
}

Answer 1

The usual problem with calling something like count++ from multiple threads (which share the count variable) is that this sequence of events can happen:

1. Thread A reads the value of count.
2. Thread B reads the value of count.
3. Thread A increments its local copy.
4. Thread B increments its local copy.
5. Thread A writes the incremented value back to count.
6. Thread B writes the incremented value back to count.

This way, the value written by thread A is overwritten by thread B, so the value is actually incremented only once.

Your code adds locks around operations 1, 2 (get) and 5, 6 (set), but that does nothing to prevent the problematic sequence of events.

What you need to do is to lock the whole operation, so that while thread A is incrementing the value, thread B can’t access it at all:

lock (progressLock)
{
    progress.CurrentCount++;
}

If you know that you will only need incrementing, you could create a method on Progress that encapsulates this.