In asp.net core controller, why does ExecutionContext.SuppressFlow() throw "AsyncFlowControl object must be used on the thread where it was created."

Question

I am attempting to suppress the flow of the execution context across asynchronous threads. I've written below code, but it throws an error -

InvalidOperationException: AsyncFlowControl object must be used on the thread where it was created.
System.Threading.AsyncFlowControl.Undo()
Web1.Controllers.ValuesController+<Get>d__0.MoveNext() in ValuesController.cs
+
                throw;
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

The error is not thrown when I debug through the code. It only occurs when I dont debug.

Sample code:

[HttpGet]
public async Task<IActionResult> Get()
{
    try
    {
        using (ExecutionContext.SuppressFlow())
        {
            await Switch.Instance.SwitchOn();
        }
    }
    catch (Exception ex)
    {
        var x = ex.Message;
        throw;
    }
    return Ok("Done!");
}

Am I going about it the wrong way?

When the continuation that was set up using `await` is executed, it may be on a different thread than the thread your code was on before `await`. — Lasse V. Karlsen, Jan 23 '19 at 12:05
Personal opinion; Unfortunately it sounds like a very broken design of `AsyncFlowControl`, given that it uses the word `Async` as part of its name and does not handle this kind of thing, it sounds like it is more thread-based than it is async (which are two different things). — Lasse V. Karlsen, Jan 23 '19 at 12:06
Seems reasonable to me. Once you've resumed on another thread, it's too late to suppress flow of execution context. — Peter Ruderman, Jan 24 '19 at 03:23

score 4 · Answer 1 · answered Jan 24 '19 at 03:15

I am attempting to suppress the flow of the execution context across asynchronous threads.

Just have to ask: Why?

I've written below code, but it throws an error

This errors happens when you restore execution context flow on a different thread that you suspended it on.

To fix this error, just don't use await within the using block:

Task task;
using (ExecutionContext.SuppressFlow())
  task = Switch.Instance.SwitchOn();
await task;

By keeping the code in the using synchronous, you are ensuring you stay on the same thread.

In asp.net core controller, why does ExecutionContext.SuppressFlow() throw "AsyncFlowControl object must be used on the thread where it was created."

1 Answers1