CODEWITHSUNDEEP
c#lambdamodal-dialogwindows-store-appsasync-await
roryok
roryok

Reputation: 9645

The 'await' operator can only be used within an async lambda expression

I've got a c# Windows Store app. I'm trying to launch a MessageDialog when one of the command buttons inside another MessageDialog is clicked. The point of this is to warn the user that their content is unsaved, and if they click cancel, it will prompt them to save using a separate save dialog.

Here's my "showCloseDialog" function:

private async Task showCloseDialog()
{
  if (b_editedSinceSave)
  {
    var messageDialog = new MessageDialog("Unsaved work! Close anyway?"
                                            , "Confirmation Message");

    messageDialog.Commands.Add(new UICommand("Yes", (command) =>
    {
      // close document
      editor.Document.SetText(TextSetOptions.None, "");
    }));

    messageDialog.Commands.Add(new UICommand("No", (command) =>
    {
      // save document
      await showSaveDialog();
    }));

    messageDialog.DefaultCommandIndex = 1;
    await messageDialog.ShowAsync();
  }
}

In VS I get a compiler error:

The 'await' operator can only be used within an async lambda expression. Consider marking this lambda expression with the 'async' modifier`

The method is marked with await. If I remove await from before showSaveDialog, it compiles (and works) but I get a warning that I really should use await

How do I use await in this context?

Upvotes: 77

Views: 73522

Answers (2)

Jhonnatan Eduardo
Jhonnatan Eduardo

Reputation: 369

In my case I needed to call an asynchronous method inside a async foreach. The solution was something like this:

await list.ForEachAsync(async item =>
{
    IEnumerable<Object> data = await _repository.Get();
});

Upvotes: 2

Boris Parfenenkov
Boris Parfenenkov

Reputation: 3279

You must mark your lambda expression as async, like so:

messageDialog.Commands.Add(new UICommand("No", async (command) =>
{
    await showSaveDialog();
}));

Upvotes: 139

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