Incorrect behavior of BackgroundWorker in Office solutions

Question

Hello,

I am developing an Excel solution where I need to do some background processing each time a sheet is modified.

My requirement is that the result of the background operation should be processed in the Excel UI (main) Thread.

I am trying use the BackgroundWorker class to execute this operation since the specification of this class is exactly what I am looking for.

However, my test shows that the RunWorkerCompleted event is not raised in the UI thread (which is the case when BackgroundWorker is used in a Windows Forms context). I have found some articles in this forum mentioning this behavior, including workarounds, but I have not seen any official statement that this is either a bug or an expected behavior of BackgroundWorker when it is used in a VSTO solution.

I could workaround this defect by using Control.Invoke to call my code. However, this is exactly what the BackgroundWorker is designed to do so I am looking for an explanation ...

This is the minimal code sample that you can add to an Excel Workbook cusomization in the code-behind file of a sheet:

 

private BackgroundWorker _worker;

 

private void OnSheetChange(Microsoft.Office.Interop.Excel.Range Target) {

  if (_worker == null) {

    TraceOperation("OnSheetChange (start worker)");

    _worker = new BackgroundWorker();

    _worker.DoWork += OnDoWork;

    _worker.RunWorkerCompleted += OnRunWorkerCompleted;

    _worker.RunWorkerAsync();

   }

}

 

private static void OnDoWork(object sender, DoWorkEventArgs e) {

  TraceOperation("Do Work");

  Thread.Sleep(2000);

}

 

private void OnRunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) {

  TraceOperation("RunWorkerCompleted");

  _worker = null;

}

 

private static void TraceOperation(string operation) {

  Trace.WriteLine(operation + " - Thread #" + Thread.CurrentThread.ManagedThreadId);

}

My question is: does BackroundWorker rely on a different SynchronizationContext when it is used within VSTO applications ? Is it an expected behavior or a bug ? Is there a workaround other than using Control.Invoke (in my case, I may not have a realized control available when I need to process the result in the UI Thread).

Thanks a lot for your help

--

Cyril

 

 

Answer 1

Hello,

Would anyone care to reply to this thread ? Reproducing the behavior that I describe should be very straightforward.

I am going to cross post this question to the "Inovate On Office" forum ...

Regards

-- Cyril

Answer 2

Hi Grimshu,

I worked on the issue and it is one of the known issues with VSTO. To workaround the issue, Before using a BackgroundWorker (e.g. before you call RunWorkerAsync on a BackgroundWorker) you need to call SetSynchronizationContext like this:
System.Threading.SynchronizationContext.SetSynchronizationContext(new WindowsFormsSynchronizationContext());

Here is the code that I tried and it resolves the issue.

TraceOperation("OnSheetChange (start worker)");

System.Threading.SynchronizationContext.SetSynchronizationContext(new WindowsFormsSynchronizationContext());

_worker = new BackgroundWorker();

_worker.DoWork += OnDoWork;

_worker.RunWorkerCompleted += OnRunWorkerCompleted;

_worker.RunWorkerAsync();

Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Regards,

Sidharth Shah

Microsoft Online Community Support

 

 

 

Answer 3

HI Grimshu and Sidharth,

I had the same problem in VSTA code inside a thirdy party application.

The workaround suggested by Sidharth does solve the problem.

Just a clarifiation: it is important to invoke

System.Threading.SynchronizationContext.SetSynchronizationContext(new WindowsFormsSynchronizationContext());

before calling the

_worker.RunWorkerAsync();

it doesn't matter to invoke it at construction time of BackgroundWorker.

Regards.

Answer 4

Hi,

this information from Sidhath is essential for understanding how to get C# 5.0 async, using Task, Task<T>, Task.Delay() and await working within a VSTO add in.

I just couldn't work out why my async control flow was being done on threads with different ManagedThreadId values. Setting the SynchronizationContext gets everything working asynchronously on the same thread - a great starting point. Maybe this should be the default?

First time your user invokes a modal dialog in the Microsoft Word UI, your VSTO add in async code will likely fail with a RPC_E_SERVERCALL_RETRYLATER exception unless you are on the same thread - as per Sidhath's code - but even better if you can avoid a background thread at all by using short lived asynchronous operations, as made pretty easy in C# 5.0.

Hopefully my combination of keywords in this reply will help someone else - I've been surfing some combination of them for ages until I found this thread.

Regards,

George.