Why Does `HttpClient.GetAsync()` Deadlock with Async/Await in .NET 4.5?

In .NET 4.5, HttpClient.GetAsync() and Async/Await cause deadlock

In .NET 4.5, when using the HttpClient class in conjunction with the async/await pattern, there may be an issue where the result of httpClient.GetAsync(...) "waits" hangs indefinitely in some cases. In the provided code, this behavior was observed in the "test5" case and raises the question of whether there is a bug in the HttpClient class or whether the API is being misused.

Cause of deadlock

Deadlocks are caused by a mismatch between the thread context and synchronization mechanism used by the async/await pattern and the inherent thread blocking behavior of certain methods. While waiting for a Task in the ASP.NET request context, this method will typically resume on the captured SynchronizationContext, ensuring that the request context is preserved.

However, in the case of test5, AsyncAwait_GetSomeDataAsync started a blocking operation, i.e. HttpClient.GetAsync, while waiting for the result of Test5Controller.Get. This blocks the thread that owns the request context, preventing processing of Task in that context from completing.

Best practices for using Async/Await and HttpClient

To avoid this deadlock situation, the following best practices are recommended:

• Use ConfigureAwait(false) in "library" async methods whenever possible. This allows the continuation to run on a normal thread pool thread, bypassing the ASP.NET request context.
• Avoid blocking Task operations; instead, utilize the async/await pattern throughout your code.

By adhering to these practices, you can achieve both of the benefits of using async/await: running continuations on a separate thread pool thread without the need for an ASP.NET request context, and keeping the controller itself async to prevent the request thread from blocking .

The above is the detailed content of Why Does `HttpClient.GetAsync()` Deadlock with Async/Await in .NET 4.5?. For more information, please follow other related articles on the PHP Chinese website!