Excel/Addin hanging when using GetAwaiter.GetResult downstream of an ExcelAsyncFunction

[Graeme Smith]

Hi

We recently ported the majority of our functions to use the ExcelAsyncFunction attribute, which gives us far superior performance and user experience.

There is one edge case, however:

If we have an ExcelAsyncFunction (or synchronous ExcelFunction) which has, downstream somewhere, a call which does GetAwaiter().GetResult(), then Excel hangs for a few minutes, before returning the "Microsoft Excel is waiting for another application to complete an OLE action" popup. 

On clicking "Cancel", the exception is

System.Runtime.InteropServices.COMException (0x80004005): Unspecified error (0x80004005 (E_FAIL))
   at System.Threading.Monitor.ObjWait(Int32 millisecondsTimeout, Object obj)
   at System.Threading.Monitor.Wait(Object obj, Int32 millisecondsTimeout)
   at System.Threading.ManualResetEventSlim.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.SpinThenBlockingWait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.InternalWaitCore(Int32 millisecondsTimeout, CancellationToken cancellationToken)

   at *.*.ProofOfHang()

Code to reproduce:

[ExcelAsyncFunction]
public static async Task<string> ProofOfHang()
{
AsyncTask().GetAwaiter().GetResult();

return "bob";
}

internal static async Task AsyncTask()
{
await Task.Delay(5000);
}

[ExcelFunction]
public static string ProofOfHang2()
{
AsyncTask().GetAwaiter().GetResult();

return "bob";
}

Both ProofOfHang and ProofOfHang2 "hang".

We register our async functions at addin load:

ExcelRegistration
.GetExcelFunctions()
.ProcessAsyncRegistrations(nativeAsyncIfAvailable: false)

In our case, we cannot, at this stage, untangle the downstream code to provide a "pure" async path.

We are using v1.7.0

Thanks

Graeme

[Govert van Drimmelen]

Hi Graeme,

 

Could you give this a try:

 

        [ExcelAsyncFunction]

        public static string NotHanging()

        {

            AsyncTask().GetAwaiter().GetResult();

 

            return "bob";

        }

 

        [ExcelAsyncFunction]

        public static string NotHanging2()

        {

            System.Threading.Thread.Sleep(5000);

            return "bob";

        }

 

A function that does not return Task<T> but is marked with [ExcelAsyncFunction] will be wrapped inside a Task.Run( …).

Your other options run synchronously until the Task itself is returned, and since you are calling GetResult() will block.

 

-Govert

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[Graeme Smith]

Thanks Govert!

Changing all the impacted method signatures from 

static async Task<T> Method()

to 

static T Method()

whilst retaining the ExcelAsyncFunction attribute solves the issue.