Why Does My HttpClient.GetAsync Call Hang When Using Await/Async in ASP.NET?

The reason why HttpClient.GetAsync(...) hangs when using Await/Async in ASP.NET

In ASP.NET, only one thread can handle a request at the same time. Although parallel processing is possible, only one thread owns the request context. This thread management is controlled by the ASP.NET SynchronizationContext.

When waiting for a Task, methods typically resume on the captured SynchronizationContext (or TaskScheduler if it does not exist). This is consistent with the expected behavior of asynchronous controller operations.

Problem in Test Case 5

The deadlock in Test5Controller.Get is due to the following sequence:

1. Call HttpClient.GetAsync in the ASP.NET request context.
2. HttpClient.GetAsync sends an HTTP request and returns an unfinished Task.
3. AsyncAwait_GetSomeDataAsync awaits a Task and returns an unfinished Task because the request is still in progress.
4. Test5Controller.Get blocks the current thread until the Task returned by AsyncAwait_GetSomeDataAsync is completed.
5. HTTP response received, HttpClient.GetAsync Task completed.
6. AsyncAwait_GetSomeDataAsync attempts to resume in the context of an ASP.NET request.
7. A deadlock occurs because the thread blocked in Test5Controller.Get still holds the request context.

Solve the problem

There are some best practices that can be implemented to avoid similar problems:

• Use ConfigureAwait(false) in library async methods to dispatch continuations outside the ASP.NET request context.
• Avoid blocking Tasks by using await instead of GetResult or Task.Wait.

Conclusion

Understanding the role of SynchronizationContext and best practices for using async/await technology can ensure efficient and deadlock-free operation when working with asynchronous code in ASP.NET.

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