Agent skill

async-patterns

Async/await patterns for the .NET 8 WPF widget host app. Use when implementing async commands, background operations, cancellation, progress reporting, or UI thread marshaling.

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SKILL.md

Async Patterns

Overview

Establish consistent async patterns that keep the UI responsive while handling long-running operations safely.

Core Principles

  1. Never block the UI thread - Use async/await, not .Result or .Wait()
  2. Always provide cancellation - Long operations should be cancellable
  3. Report progress - Users should see feedback on long operations
  4. Handle exceptions - Async exceptions need explicit handling

Patterns

1. Async Commands (RelayCommand)

csharp
[RelayCommand]
private async Task LoadDataAsync(CancellationToken cancellationToken)
{
    IsLoading = true;
    try
    {
        Data = await _service.GetDataAsync(cancellationToken);
    }
    finally
    {
        IsLoading = false;
    }
}

2. Cancellation Support

csharp
private CancellationTokenSource? _cts;

[RelayCommand]
private async Task StartOperationAsync()
{
    _cts?.Cancel();
    _cts = new CancellationTokenSource();
    
    try
    {
        await LongRunningOperationAsync(_cts.Token);
    }
    catch (OperationCanceledException)
    {
        // User cancelled - this is expected
    }
}

[RelayCommand]
private void Cancel()
{
    _cts?.Cancel();
}

3. Progress Reporting

csharp
[ObservableProperty]
private int _progress;

[RelayCommand]
private async Task ProcessItemsAsync(CancellationToken cancellationToken)
{
    var items = await GetItemsAsync(cancellationToken);
    
    for (int i = 0; i < items.Count; i++)
    {
        cancellationToken.ThrowIfCancellationRequested();
        await ProcessItemAsync(items[i], cancellationToken);
        Progress = (i + 1) * 100 / items.Count;
    }
}

4. UI Thread Marshaling

csharp
// When updating UI from background thread
Application.Current.Dispatcher.Invoke(() =>
{
    Items.Add(newItem);
});

// Prefer async version when possible
await Application.Current.Dispatcher.InvokeAsync(() =>
{
    Items.Add(newItem);
});

5. Fire-and-Forget (Use Sparingly)

csharp
// Only for truly fire-and-forget scenarios
_ = Task.Run(async () =>
{
    try
    {
        await BackgroundWorkAsync();
    }
    catch (Exception ex)
    {
        Log.Error(ex, "Background work failed");
    }
});

Definition of Done (DoD)

  • Long-running operations use async/await
  • No .Result or .Wait() calls on UI thread
  • Operations > 100ms have loading indicators
  • User-initiated operations support cancellation
  • Progress shown for batch operations
  • Exceptions logged and user-notified appropriately
  • Dispatcher used for cross-thread UI updates

Anti-Patterns

❌ Don't ✅ Do
task.Result await task
task.Wait() await task
async void methods async Task methods
Swallowing exceptions Log and notify user
Blocking UI for network Show loading, use async

Key Components

Component Purpose
[RelayCommand] CommunityToolkit async command generation
CancellationTokenSource Cancellation coordination
IProgress<T> Progress reporting interface
Dispatcher.InvokeAsync UI thread marshaling

Task Scheduling

For background work that shouldn't block commands:

csharp
// Schedule periodic background work
private readonly PeriodicTimer _timer = new(TimeSpan.FromMinutes(5));

private async Task StartBackgroundRefreshAsync(CancellationToken stoppingToken)
{
    while (await _timer.WaitForNextTickAsync(stoppingToken))
    {
        try
        {
            await RefreshDataAsync(stoppingToken);
        }
        catch (Exception ex) when (ex is not OperationCanceledException)
        {
            Log.Warning(ex, "Background refresh failed");
        }
    }
}

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