Why Does Using `async void` in ASP.NET Often Result in 'An asynchronous module or handler completed while an asynchronous operation was still pending'?

Async Void and ASP.Net: Understanding the Exception

When using async void methods in ASP.Net applications, you may encounter an exception stating, "An asynchronous module or handler completed while an asynchronous operation was still pending." This exception arises because async void increments the count of outstanding operations upon invocation and decrements it upon completion.

In contrast, async Task methods return a Task object, allowing the ASP.Net framework to track the asynchronous operation and handle completion gracefully.

Why Task Helps

By returning Task, you enable the framework to:

• Monitor the progress of the operation and ensure it completes before ending the request.
• Handle any exceptions that may arise during the operation and propagate them to the caller.
• Manage the synchronization context and ensure that asynchronous operations are run on the appropriate thread.

Async Void and Fire-and-Forget

While async void methods may appear to implement a fire-and-forget approach, this is inaccurate in the context of ASP.Net. The framework actively tracks these methods, raising an exception if they complete prematurely.

To achieve a true fire-and-forget behavior, use async Task methods and ignore the returned Task. However, premature return from requests in ASP.Net is generally discouraged due to potential issues.

Additional Notes

• Microsoft designed ASP.Net's async support to retain backward compatibility, explaining the use of async void in some scenarios.
• WebForms utilizes async void event handlers, while MVC/WebAPI/SignalR rely on async Task.
• Mixing async Task and async void is not supported, as it can lead to the exception described.

The above is the detailed content of Why Does Using `async void` in ASP.NET Often Result in 'An asynchronous module or handler completed while an asynchronous operation was still pending'?. For more information, please follow other related articles on the PHP Chinese website!