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Problem with my Async/Await implementation

Hi everyone,

I'm encountering some persistent issues with my async and await implementation in a UWP application. I'm trying to perform a network request and update the UI, but I'm facing a System.InvalidOperationException saying "The application called an interface that was marshalled for a different thread."

Here's a simplified version of my code:

public async Task LoadDataAsync()
{
    try
    {
        var data = await GetDataFromApiAsync();
        // Problematic UI update here?
        MyTextBlock.Text = data.Result;
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine($"Error: {ex.Message}");
    }
}

private async Task<ApiResponse> GetDataFromApiAsync()
{
    // ... network request logic ...
    await Task.Delay(1000); // Simulate network latency
    return new ApiResponse { Result = "Data loaded successfully!" };
}
                        

I've tried using await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => { ... }); but it doesn't seem to resolve the issue reliably. Any suggestions on how to correctly handle UI updates from async operations in UWP?

Thanks in advance!

Re: Problem with my Async/Await implementation

Hello John,

The exception "The application called an interface that was marshalled for a different thread" is a classic indicator of trying to update UI elements from a background thread. In UWP (and WPF), UI elements are typically tied to the UI thread they were created on.

While Dispatcher.RunAsync is the correct approach, it's crucial to wrap the entire UI update block. Let's refine your code:

public async Task LoadDataAsync()
{
    try
    {
        var data = await GetDataFromApiAsync();
        await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(
            Windows.UI.Core.CoreDispatcherPriority.Normal,
            () =>
            {
                MyTextBlock.Text = data.Result;
            }
        );
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine($"Error: {ex.Message}");
    }
}
                        

Notice that the `MyTextBlock.Text = data.Result;` line is now inside the `RunAsync` lambda. This ensures that the assignment happens on the UI thread.

Also, ensure your `GetDataFromApiAsync` method itself doesn't accidentally perform UI operations. If it does, it would need similar dispatcher handling or be designed to only return data.

Let me know if this resolves your issue!

Re: Problem with my Async/Await implementation

Alice, thank you so much!

You were absolutely right. I had incorrectly assumed that simply calling await on the async method would keep the subsequent code on the UI thread. Wrapping the actual UI update within Dispatcher.RunAsync did the trick. I've applied your suggested code, and the exception is gone. The UI is updating as expected now.

I appreciate your clear explanation and the correct code snippet. This has been a great help!