VB.NET: Async method state machine "losing" locals - Release only (Async/Await + Try/Finally + For)

[KirillShlenskiy]

I've come across a very strange NullReferenceException issue which only manifests itself when building in Release configuration (with Advanced Compile Options -> Enable optimizations enabled).

This occurs in Async methods (both Task-returning and Async Sub) which have a Try/Finally block where the Try contains a For loop, which in turn contains an Await statement. When a local variable pointing to a reference type instance is declared before such a Try/Finally and accessed inside the Finally, an "impossible" NullReferenceException is thrown.

Here is a complete Console application demonstrating the issue when built with VS2015 (targeting .NET4.5, Release):

Module Module1

  Sub Main()
    TestAsync().Wait()
  End Sub

  Async Function TestAsync() As Task
    Dim obj As New Object

    Try
      For i = 0 To 0
        Await Task.Yield()
      Next
    Finally
      obj.GetType() ' Throws System.NullReferenceException
    End Try
  End Function

End Module

Analysis

Looking at the Release binary with ILDASM, the compiler-generated VB$StateMachine_1_TestAsync is missing the following field definition:

.field assembly object $VB$ResumableLocal_obj$0

Here are the relevant bits of MoveNext IL in Debug (works fine):

ldarg.0
newobj     instance void [mscorlib]System.Object::.ctor()
call       object [mscorlib]System.Runtime.CompilerServices.RuntimeHelpers::GetObjectValue(object)
stfld      object ConsoleRepro.Module1/VB$StateMachine_1_TestAsync::$VB$ResumableLocal_obj$0
... Await ...
ldarg.0
ldfld      object ConsoleRepro.Module1/VB$StateMachine_1_TestAsync::$VB$ResumableLocal_obj$0
callvirt   instance class [mscorlib]System.Type [mscorlib]System.Object::GetType()

VS Release (throws):

newobj     instance void [mscorlib]System.Object::.ctor()
call       object [mscorlib]System.Runtime.CompilerServices.RuntimeHelpers::GetObjectValue(object)
stloc.1
... Await ...
ldloc.1
callvirt   instance class [mscorlib]System.Type [mscorlib]System.Object::GetType()

The above works fine if the Await completes synchronously, but if not - the local is uninitialized after Await, hence the NullReferenceException.

Environment

The above has been reproduced using:

• Visual Studio 2015 Professional (14.0.24720.00 Update 1)
• Visual Studio 2015 Community (14.0.23107.0)

Addendum

If obj is IDisposable, you can replace Try/Finally with a Using statement and then the problem is even worse due to the null check auto-inserted by the compiler before the obj.Dispose call, meaning that NullReferenceException is never thrown - Dispose simply doesn't get called, i.e.

Async Function TestAsync() As Task
  Using obj As New MyDisposable ' IDisposable
    For i = 0 To 0
      Await Task.Yield()
    Next
  End Using ' obj.Dispose *not* called
End Function

[roken]

This also impacts the state machine generated for iterator methods, and the characteristics you describe are identical: http://stackoverflow.com/questions/35568392/vb-net-iterator-function-loses-local-variables?noredirect=1#comment58844116_35568392

[KirillShlenskiy]

Thanks @roken

I'm not certain that this is the same issue but I suppose it might be.

I've done a bit of digging and found that the asynchronous variable capture issue is, apparently, already fixed:

#7693

I have confirmed that the code I posted in the opening comment for this issue (9001) compiles and executes as expected using the latest Roslyn source, so I intend to self-close this issue.

As for the iterator problem, I have answered your SO question with an example unit test which passes using the latest version of Roslyn, so I am assuming that it is also fixed.

[KirillShlenskiy]

Thanks @davkean

I am now closing this issue as I have confirmed that it's been fixed in the latest codebase.

[davkean]

Thanks for the followup @KirillShlenskiy, appreciate it.

[davkean]

Dupe of #7693