xecution stops silently after Await inside Select Case (Finally not run)

madru

Hi Garry,

another async issue, found while porting the report screens of a database app. Execution silently stops after an Await inside a Select Case body: no exception, and the Finally block of the enclosing Try doesn't run.

Setup

A PopupMenu SelectionChanged handler calls an Async Sub (plain call, not awaited). The method is guarded by a Boolean that is reset in Finally:

  If _loadingReport Then Return
  _loadingReport = True
  Try
    Var data As DBAnswer = Nothing
    Var rep As BarReport = Nothing

    Select Case cbReport.SelectedRowIndex
      Case 1
        data = Await GetTotalByColorByMonth(year, lack)     # works
      ...
      Case 7
        Print("7: before Await")
        rep = Await CreateBarData1(year, lack)             # Async Function As BarReport
        Print("7: after Await")
      Else
        data = Await GetInspection(Nothing, lack)          # works
    End Select
    Print("after Select")
    ...
  Catch ex As Exception
    Print("LRA exception: " + ex.GetType() + ": " + ex.Message)
  Finally
    Print("finally, resetting guard")
    _loadingReport = False
  End Try
  Print("LoadReportAsync done")
End Sub

Observed output for case 7

LRA start, guard=False
7: before Await
CBD1 start
CBD1 end of Try
CBD1 returning
7: after Await

...and nothing more. No "after Select", no exception (even with Catch ex As Exception), no "finally". On the next selection the guard is still True, so every later call returns immediately and the app looks frozen.

For the other cases (1-6, Else), which also Await inside their Case body, the output is complete:

after Select
finally, resetting guard
LoadReportAsync done

The awaited function itself completes normally (the "CBD1" lines are printed inside it; it contains a Try/Catch with an Await inside).

Workaround

Moving the Await out of the Case body fixes it. The case only sets a number, and the chart is loaded after End

objo
Select:

      Case 7
        chartNo = 7
    End Select
    If chartNo > 0 Then
      Var rep As BarReport = Await LoadChartReport(chartNo, year, lack)
      ...
    End If

With that, case 7 prints "after Select", "finally" and "done" like the others.

I haven't yet reduced this to a minimal standalone project, and I can't tell exactly what distinguishes case 7 from the working cases (it assigns a class instance returned by an Async Function that itself contains Try/Catch around Awaits; the working cases assign a DBAnswer from functions without Try). Happy to send the full project or build a small repro if that helps.

Related observations (possibly the same root cause)

Return inside a Try with Finally in an Async Sub, after an Await, appeared to leave the method without running Finally (the same guard stayed True). Removing the Return fixed it.
The Async/Await docs show Var task As Task(Of String) = GetName() to start a task without awaiting. For me, Var t1 As Task(Of DBAnswer) = client.GetRecordAsync(...) fails with "Cannot assign 'DBAnswer' to variable of type 'Task(Of DBAnswer)'".
The docs' fire-and-forget discard _ = SomeAsyncSub() fails with "Undefined identifier '_'". A plain call SomeAsyncSub() works.

Thanks again for the quick turnaround on the other reports!

Best regards,
Guido

Garry

Thank you for this exceptionally well-written report. It contained three genuine compiler bugs, and every one of your related observations was spot on. All three are fixed and will ship in the next release.

1. Finally blocks were skipped on early exits from Try (your observation 1)

A Return inside a Try ... Finally - including after an Await - jumped straight out without running the Finally. The same applied to Exit Do, Exit For, Exit While and Continue leaving a Try block. Finally now runs on every early exit path, which is exactly what protects your busy-guard Boolean pattern: the guard can no longer stay True and freeze later calls. While fixing this I also found and fixed a related latent defect where an Exit For inside a Try could crash the VM outright.

2. Async calls now produce a real Task (your observation 2)

Previously a call to an Async method without Await didn't give you a usable value, so the documented pattern failed with errors like "Cannot assign 'DBAnswer' to variable of type 'Task(Of DBAnswer)'". Outside an Await, a call to an Async Sub now evaluates to a Task and an Async Function to a Task(Of T), so storing the task and awaiting it later works as documented.

3. The _ = discard (your observation 3)

_ = SomeAsyncSub() now compiles and discards the result as the docs promise. The warning it's documented to suppress - "Result of async call is not awaited. Exceptions will be silently lost." - turned out never to have been implemented either, so bare fire-and-forget calls now produce that warning, and _ = MethodName() is the way to acknowledge the intent and silence it. One heads-up: _ is now reserved, so it can no longer be used as a variable, parameter or member name. Public tracking for this one: https://feedback.objo.dev/bug/1595

About the main symptom

I rebuilt the reported shape several ways - the PopupMenu.SelectionChanged handler, the guard pattern, a Select Case with an Await inside a Case body, and the awaited function containing its own Try/Catch around Awaits - and every reconstruction ran to completion, so I couldn't reproduce the silent stop itself. Your workaround (setting chartNo in the Case and awaiting LoadChartReport after the End Select) is a good one regardless. With the fixes above in place, the direct Await inside the Select Case body should now behave correctly too - but if you still see execution stop silently after an Await once the next release is out, please do take me up on your offer of the project, because I'd very much like to chase any remaining shape down.