Starting Tasks In foreach Loop Uses Value of Last Item [duplicate]

by

You’re closing over the loop variable. Don’t do that. Take a copy instead:

foreach (string path in paths)
{
    string pathCopy = path;
    var task = Task.Factory.StartNew(() =>
        {
            Boolean taskResult = ProcessPicture(pathCopy);
            return taskResult;
        });
    // See note at end of post
    task.ContinueWith(t => result &= t.Result);
    tasks.Add(task);
}

Your current code is capturing path – not the value of it when you create the task, but the variable itself. That variable changes value each time you go through the loop – so it can easily change by the time your delegate is called.

By taking a copy of the variable, you’re introducing a new variable each time you go through the loop – when you capture that variable, it won’t be changed in the next iteration of the loop.

Eric Lippert has a pair of blog posts which go into this in a lot more detail: part 1; part 2.

Don’t feel bad – this catches almost everyone out 🙁

Note about this line:

task.ContinueWith(t => result &= t.Result);

As pointed out in comments, this isn’t thread-safe. Multiple threads could execute it at the same time, potentially stamping on each other’s results. I haven’t added locking or anything similar as it would distract from the main issue that the question is interested, namely variable capture. However, it’s worth being aware of.

More Related Contents:

  1. What is the difference between task and thread?
  2. What is the difference between Task.Run() and Task.Factory.StartNew()
  3. When to use Task.Delay, when to use Thread.Sleep?
  4. Execute task in background in WPF application
  5. Does the use of async/await create a new thread?
  6. When to use BlockingCollection and when ConcurrentBag instead of List?
  7. Is it considered acceptable to not call Dispose() on a TPL Task object?
  8. Multiple Parallel.ForEach loops in .Net
  9. Proper way to implement a never ending task. (Timers vs Task)
  10. How to create a task (TPL) running a STA thread?
  11. How to put a task to sleep (or delay) in C# 4.0?
  12. Is Task.Factory.StartNew() guaranteed to use another thread than the calling thread?
  13. Cancelling a pending task synchronously on the UI thread
  14. Does using Tasks (TPL) library make an application multithreaded?
  15. The current SynchronizationContext may not be used as a TaskScheduler
  16. ConfigureAwait pushes the continuation to a pool thread
  17. ThreadPool.QueueUserWorkItem vs Task.Factory.StartNew
  18. Process queue with multithreading or tasks
  19. How to achieve remove_if functionality in .NET ConcurrentDictionary
  20. Calling async methods from non-async code
  21. SynchronizationLockException on Monitor.Exit when using await
  22. Why CancellationToken is separate from CancellationTokenSource?
  23. Need to understand the usage of SemaphoreSlim
  24. Should i use ThreadPools or Task Parallel Library for IO-bound operations
  25. Faulted vs Canceled task status after CancellationToken.ThrowIfCancellationRequested
  26. Synchronous implementation of interface that returns Task
  27. The lack of non-capturing Task.Yield forces me to use Task.Run, why follow that?
  28. How to properly parallelise job heavily relying on I/O
  29. Wrapping ManualResetEvent as awaitable task
  30. Calling TaskCompletionSource.SetResult in a non blocking manner

Leave a Comment