Why Does Async Void Cause `System.InvalidOperationException` in ASP.NET?

Understanding Async Void in ASP.Net: Outstanding Operations Count

Issue: Async void methods within ASP.Net applications raise the following exception:

System.InvalidOperationException: An asynchronous module or handler 
completed while an asynchronous operation was still pending

Explanation:

Asynchronous methods in ASP.Net increment the count of outstanding operations upon invocation and decrement it upon completion. Async void methods directly notify the ASP.Net SynchronizationContext, bypassing the MVC framework.

Task Return vs. Async Void:

Unlike async void methods, returning a Task from asynchronous methods:

• Allows the framework to manage the returned task and await its completion
• Ensures proper synchronization with the request lifecycle
• Avoids the exception caused by returning early from requests (a common practice with async void)

In the example provided:

• ThisPageWillLoad() uses async Task, which works as expected because the MVC framework awaits the returned task.
• ThisPageWillNotLoad() uses async void, causing the exception because ASP.Net expects the method to be complete before the page finishes loading, while the async void method has not yet completed.

"Fire and Forget" Misconception:

Async void methods should not be considered "fire and forget" in ASP.Net. While they may appear to be fire-and-forget in some scenarios, the error handling semantics indicate that they are not truly forgetful.

Additional Considerations:

• Returning Task and using async void simultaneously is not supported.
• Some third-party libraries may use async void, but it's essential to avoid using such libraries as they can lead to complications.
• If you need to use TAP over EAP, consider TAP-over-APM or wrapping the EAP component with Task.Run.

The above is the detailed content of Why Does Async Void Cause `System.InvalidOperationException` in ASP.NET?. For more information, please follow other related articles on the PHP Chinese website!