Why Does HttpClient.GetAsync(...) Deadlock When Using Await/Async in .NET 4.5?

Deadlock problem using async/await and HttpClient.GetAsync(...) in .NET 4.5

Question:

Using the async/await language features with HttpClient.GetAsync(...) in .NET 4.5 may cause deadlocks under certain circumstances. Specifically, waiting for the result of HttpClient.GetAsync(...) can cause a deadlock in the following situations:

1. The thread blocks waiting for the task to complete (for example, using GetResult).
2. Receive HTTP response and complete the task.

Explanation:

The deadlock is due to the use of SynchronizationContext, which ensures that only one request can be processed at a time in ASP.NET. When using await, the method resumes on the captured SynchronizationContext, in this case the ASP.NET request context. However, if the thread blocks waiting for the task to complete, it will be unable to resume the method in the ASP.NET request context, resulting in a deadlock.

Solution:

To avoid this deadlock, follow these best practices:

1. Use ConfigureAwait(false) in "library" async methods whenever possible.
2. Avoid blocking tasks. Use await instead of GetResult, Task.Result, and Task.Wait.

By following these guidelines, you can ensure that the continuation of the AsyncAwait_GetSomeDataAsync method runs on a background thread and avoid blocking the ASP.NET request thread, thereby preventing deadlocks.

More information:

• Introduction to async/await
• Async/Await FAQ
• MSDN Forum Post
• Demo by Stephen Toub
• Demo by Lucian Wischik

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