Using HttpContext.Current in WebApi is dangerous because of async

by

HttpContext.Current gets the current context by Thread (I looked into the implementation directly).

It would be more correct to say that HttpContext is applied to a thread; or a thread “enters” the HttpContext.

Using HttpContext.Current inside of async Task is not possible, because it can run on another Thread.

Not at all; the default behavior of async/await will resume on an arbitrary thread, but that thread will enter the request context before resuming your async method.

The key to this is the SynchronizationContext. I have an MSDN article on the subject if you’re not familiar with it. A SynchronizationContext defines a “context” for a platform, with the common ones being UI contexts (WPF, WinPhone, WinForms, etc), the thread pool context, and the ASP.NET request context.

The ASP.NET request context manages HttpContext.Current as well as a few other things such as culture and security. The UI contexts are all tightly associated with a single thread (the UI thread), but the ASP.NET request context is not tied to a specific thread. It will, however, only allow one thread in the request context at a time.

The other part of the solution is how async and await work. I have an async intro on my blog that describes their behavior. In summary, await by default will capture the current context (which is SynchronizationContext.Current unless it is null), and use that context to resume the async method. So, await is automatically capturing the ASP.NET SynchronizationContext and will resume the async method within that request context (thus preserving culture, security, and HttpContext.Current).

If you await ConfigureAwait(false), then you’re explicitly telling await to not capture the context.

Note that ASP.NET did have to change its SynchronizationContext to work cleanly with async/await. You have to ensure that the application is compiled against .NET 4.5 and also explicitly targets 4.5 in its web.config; this is the default for new ASP.NET 4.5 projects but must be explicitly set if you upgraded an existing project from ASP.NET 4.0 or earlier.

You can ensure these settings are correct by executing your application against .NET 4.5 and observing SynchronizationContext.Current. If it is AspNetSynchronizationContext, then you’re good; if it’s LegacyAspNetSynchronizationContext, then the settings are wrong.

As long as the settings are correct (and you are using the ASP.NET 4.5 AspNetSynchronizationContext), then you can safely use HttpContext.Current after an await without worrying about it.

More Related Contents:

  1. What is the difference between concurrency, parallelism and asynchronous methods?
  2. How to create threads in nodejs
  3. How to articulate the difference between asynchronous and parallel programming?
  4. Asynchronous vs Multithreading – Is there a difference?
  5. How Do I Choose Between the Various Ways to do Threading in Delphi?
  6. Async WebApi Thread.CurrentCulture
  7. Spring @Async limit number of threads
  8. Does an asynchronous call always create/call a new thread?
  9. Why is setState in reactjs Async instead of Sync?
  10. How should I unit test multithreaded code?
  11. Understanding the Event Loop
  12. Difference between -pthread and -lpthread while compiling
  13. JavaFX2: Can I pause a background Task / Service?
  14. what is the correct way to implement a QThread… (example please…)
  15. CoreData: warning: Unable to load class named
  16. How to cancel Future in Scala?
  17. What is thread contention?
  18. What is a “thread” (really)?
  19. Benefits of using async and await keywords
  20. Grasping the Node JS alternative to multithreading
  21. How, if at all, do Erlang Processes map to Kernel Threads?
  22. Number of threads used by Go runtime
  23. What thread runs the code after the `await` keyword?
  24. Objective-C Asynchronous Web Request with Cookies
  25. fork in multi-threaded program
  26. Thread references require static lifetime?
  27. Passing arguments to std::async by reference fails
  28. Is it better to use TThread’s “Synchronize” or use Window Messages for IPC between main and child thread?
  29. How do I create a custom SynchronizationContext so that all continuations can be processed by my own single-threaded event loop?
  30. What happens when different CPU cores write to the same RAM address without synchronization?

Leave a Comment