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c#excelasync-awaitvstoadd-in
Max Sinev
Max Sinev

Reputation: 6019

Incorrect async/await working, Excel events in Excel Application Level Add-in

I faced a problem with using async /await in my add-in application, I have custom task pane and user can download some data with it, so there are problems:

  1. After awaited operation completed the control is not returned to previous execution context. I created example with windows forms and paste the same method with awaiting and it works fine - I can use form controls after await operation, but not in add-in, I have to call custom task pane Invoke method. Simple example:

    private void SearchPaneButton_Click(object sender, EventArgs e)
{
    Search();
}

private async void Search()
{
    var searchText = SearchTextBox.Text;
    SearchPaneButton.Text = "Loading…";
    var data = await new DataServiceClient().GetDataAsync(searchText);
    this.Invoke((MethodInvoker)(() =>
    {
        SearchPaneButton.Text = "Search";
        ToggleWorkbookEvents();
    }));
}

  2. In my add-in I need to handle some events - worksheet changed, worksheet activated/deactivated, but these events disappear and isn't fired after await operation and I have to use something like "toggling"(remove+add) events after each await using. Simple example:

    private void ToggleWorkbookEvents()
{
    var excel = Globals.MyAddIn.Application;
    //need to ensure event will not be set twice
    excel.SheetActivate -= CheckSheetForData;
    excel.SheetActivate += CheckSheetForData;
    if (excel.ActiveWorkbook != null)
    {
        var activeSheet = Globals.MyAddIn.GetActiveWorksheet();
        //need to ensure event will not be set twice
        activeSheet.Change -= Worksheet_Changed;
        activeSheet.Change += Worksheet_Changed;
    }
}

So maybe vsto framework can not support new features(like async await) of last versions .NET Framework?

Upvotes: 9

Views: 3989

Answers (2)

mmokrzycki
mmokrzycki

Reputation: 82

When it comes to your second question - events being unbound from Worksheet, Workbook etc. you have to store the reference to the target object as a property or field of class that binds those events. For example in your ToggleWorkbookEvents method you reference Application as a local variable, you could try to store it like that:

private Microsoft.Office.Interop.Excel.Application _application = Globals.MyAddIn.Application;

I believe this happens due to target event handlers being removed during garbage collection.

I base this entry on an answer by Peter: https://stackoverflow.com/a/58524311/6757641

Upvotes: 0

Stephen Cleary
Stephen Cleary

Reputation: 457147

This is a long-standing issue with Office plugins: they don't provide a SynchronizationContext.

As I mention on my blog, you can work around this by ensuring you have a proper SynchronizationContext. This is a bit hacky, but it works:

private async void SearchPaneButton_Click(object sender, EventArgs e)
{
  if (SynchronizationContext.Current == null)
    SynchronizationContext.SetSynchronizationContext(new WindowsFormsSynchronizationContext());
  await SearchAsync();
}

private async Task SearchAsync()
{
  var searchText = SearchTextBox.Text;
  SearchPaneButton.Text = "Loading…";
  var data = await new DataServiceClient().GetDataAsync(searchText);
  SearchPaneButton.Text = "Search";
  ToggleWorkbookEvents();
}

As far as the "toggling events" problem goes, I have no idea.

Upvotes: 19

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