Why Does `HttpClient.GetAsync(...)` Hang When Using `await`/`async` in ASP.NET?

ASP.NET async/await and HttpClient.GetAsync(...) Deadlocks: A Solution

The Problem: Using HttpClient.GetAsync(...) within an ASP.NET async method can lead to deadlocks. This happens because ASP.NET's single-threaded request processing model conflicts with the asynchronous nature of HttpClient. When await pauses execution, the thread is released, but resuming the task might attempt to reacquire the same thread already occupied by another request, causing a standstill.

Understanding the Deadlock:

1. httpClient.GetAsync returns an incomplete Task.
2. await suspends the current thread until the Task finishes.
3. The Task tries to resume within the ASP.NET request context, but the context is already in use.
4. A deadlock results.

Key Solutions:

1. ConfigureAwait(false): This crucial method prevents the continuation of the Task from being scheduled back onto the original context (e.g., the ASP.NET request thread). This eliminates the potential for a context-related deadlock.

2. Avoid Blocking: Always use await instead of methods like GetResult() which block synchronously on the Task, directly leading to deadlocks.

Code Examples:

Problematic Code:

public async Task<string> GetSomeDataAsync()
{
    var httpClient = new HttpClient();
    var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead);
    return result.Content.Headers.ToString();
}

Corrected Code:

public async Task<string> GetSomeDataAsync()
{
    var httpClient = new HttpClient();
    var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false);
    return result.Content.Headers.ToString();
}

Further Reading:

For a deeper understanding of asynchronous programming in .NET and how to avoid deadlocks, refer to these resources:

• Async/await Introduction
• Async/await FAQ (Note: MSDN is archived, but the information may still be accessible via web archives)
• Await, UI, and Deadlocks (Note: MSDN is archived, but the information may still be accessible via web archives)
• MSDN Forum Discussion on Deadlocks (Note: MSDN is archived, but the information may still be accessible via web archives)
• Stephen Toub's Deadlock Demonstration (Note: MSDN is archived, but the information may still be accessible via web archives)
• Lucian Wischik's Deadlock Demonstration

By consistently using ConfigureAwait(false) in your asynchronous methods, you can effectively prevent these deadlocks and ensure the smooth operation of your ASP.NET applications.

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