Question

I’ve had an issue a bit like this occasionally with Task.Run()/Task.Factory.StartNew(), so I think there must be something very basic I’m not understanding about its use (for those who like curly braces, my apologies for having to muddle through VB):

I have an async method with the following code:

       Try
        Using context As New ECOSSContext


            context.Meters.Attach(entity)


            Await Task.Run(Sub() _
                               context.SaveChanges() _
                               )

            'ANY CODE HERE NEVER GETS FIRED


        End Using

        Return True

    Catch ex As Exception
        Return False
    End Try

It is called from a synchronous method like so:

Dim result = ModelService.MeterResolution.SaveMeter(data).Result

From what I understood this should wait until the method SaveMeter() returns with a result. And the async method I laid out previously should wait on the Task.Run line until it returns, and then continues (hence the await). Instead, what happens is that as soon as the Await line is hit, the callstack seems to exit. The SaveChanges() succeeds, but I’m unable to inform the client of this fact.

I use the TPL relatively frequently in a different application and haven’t had these issues, but there’s no method like context.SaveChanges() I need to use.

Any suggestions are appreciated. I hope i’m not just mis-reading the documentation.

Answer 1

The problem is actually due to your use of Result. Calling async code from synchronous code can be quite tricky.

I explain the full reasons for this deadlock on my blog. In short, there’s a “context” that is saved by default at the beginning of each await and used to resume the method.

So if this is called in a UI or ASP.NET context, when the await completes, the async method tries to re-enter that context to continue executing. Unfortunately, code using Result (or Wait) will block a thread in that context, so the async method cannot complete.

The guidelines to avoid this are:

1. Use ConfigureAwait(continueOnCapturedContext: false) as much as possible. This enables your async methods to continue executing without having to re-enter the context.
2. Use async all the way. Use await instead of Result or Wait.