ConcurrentDictionary Pitfall – Are delegates factories from GetOrAdd and AddOrUpdate synchronized?

by

Yes, you are right, the user delegates are not synchronized by ConcurrentDictionary. If you need those synchronized it is your responsibility.

The MSDN itself says:

Also, although all methods of ConcurrentDictionary are
thread-safe, not all methods are atomic, specifically GetOrAdd and
AddOrUpdate. The user delegate that is passed to these methods is
invoked outside of the dictionary’s internal lock. (This is done to
prevent unknown code from blocking all threads.)

See “How to: Add and Remove Items from a ConcurrentDictionary

This is because the ConcurrentDictionary has no idea what the delegate you provide will do or its performance, so if it attempted lock around them, it could really impact performance negatively and ruin the value of the ConcurrentDictionary.

Thus, it is the user’s responsibility to synchronize their delegate if that is necessary. The MSDN link above actually has a good example of the guarantees it does and does not make.

More Related Contents:

  1. Create Dynamic Threads in c# [closed]
  2. Why we need Thread.MemoryBarrier()?
  3. Do I need to lock or mark as volatile when accessing a simple boolean flag in C#?
  4. thread with multiple parameters
  5. Event handlers not thread safe? [duplicate]
  6. How to correctly read an Interlocked.Increment’ed int field?
  7. Thread Safe Properties in C#
  8. Is it possible to observe a partially-constructed object from another thread?
  9. Automating the InvokeRequired code pattern
  10. How do I abort/cancel TPL Tasks?
  11. Cross-thread operation not valid [duplicate]
  12. What does a lock statement do under the hood?
  13. Concurrent HashSet in .NET Framework?
  14. BackgroundWorker RunWorkerCompleted Event
  15. How do I run a simple bit of code in a new thread?
  16. Are C# arrays thread safe?
  17. How to wait for a BackgroundWorker to cancel?
  18. C# Thread Termination and Thread.Abort()
  19. How to create a task (TPL) running a STA thread?
  20. Cancelling a pending task synchronously on the UI thread
  21. ConfigureAwait pushes the continuation to a pool thread
  22. Are IEnumerable Linq methods thread-safe?
  23. Creating threads – Task.Factory.StartNew vs new Thread()
  24. C#/.NET analysis tool to find race conditions/deadlocks
  25. C# Windows Forms Application – Updating GUI from another thread AND class?
  26. How to interrupt Console.ReadLine
  27. Error: Must create DependencySource on same Thread as the DependencyObject even by using Dispatcher
  28. Locking pattern for proper use of .NET MemoryCache
  29. Unit test for thread safe-ness?
  30. The lack of non-capturing Task.Yield forces me to use Task.Run, why follow that?

Leave a Comment