Error 500 in Bot Framework, InvalidOperationException: An asynchronous module or handler completed while async operation pending

Question

I have a bot built with C# / MS Bot Framework which runs on Azure. It works normally, but HTTP logs in Azure show a large number of failed HTTP requests, about 20% of total.

When I enabled failed request tracing, that's what I saw in the logs:

URL: /api/messages

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace

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

I could not extract any more details from the logs, including the stack trace (it is empty).

The bot also works normally in the emulator. The failed requests appear when talking to the bot through Facebook, even though the user can interact with the bot normally despite the seemingly failed requests.

I can't figure out which part of the code might even cause this and my other bots do not seem to have this issue.

I suspect it could be that I'm using context.Done(this) to terminate a dialog, but this seems to be a normal practice to use this.

Any pointers or ideas which part of the code might cause this, or how to extract more details about the error, will be appreciated. Thanks.

Answer 1

Without seeing the code is a bit hard to exactly know what's going on, however that exception is pretty common in the async/await world. Usually it means there's a race condition between your module/handler completing and the completion of an async method.

I would look for async methods called but not being awaited.

Some related readings:

• ASP.NET Controller: An asynchronous module or handler completed while an asynchronous operation was still pending
• Web Api + HttpClient: An asynchronous module or handler completed while an asynchronous operation was still pending