WPF Dispatcher Threading

Priority Levels (High to Low)

Priority Value Use Case

Send 10 Synchronous (avoid - deadlock risk)

Normal 9 Standard operations

DataBind 8 Binding updates

Render 7 Rendering operations

Loaded 6 Loaded event handlers

Input 5 User input processing

Background 4 Background tasks (UI stays responsive)

ContextIdle 3 After context operations

ApplicationIdle 2 App idle (cache cleanup)

SystemIdle 1 System idle

Inactive 0 Disabled

Common Patterns

Background Work (UI Responsive)

await Dispatcher.InvokeAsync(() => { // This runs between input processing ProcessNextChunk(); }, DispatcherPriority.Background);

After Render Complete

await Dispatcher.InvokeAsync(() => { // Runs after layout/render ScrollIntoView(lastItem); }, DispatcherPriority.Loaded);

Idle Cleanup

Dispatcher.InvokeAsync(() => { // Low priority cleanup ClearUnusedCache(); }, DispatcherPriority.ApplicationIdle);

Yielding Pattern

Keep UI responsive during long operations:

public async Task ProcessLargeDataAsync(IList<Item> items) { for (int i = 0; i < items.Count; i++) { Process(items[i]);

    // Yield every 100 items to process pending input
    if (i % 100 == 0)
    {
        await Dispatcher.Yield(DispatcherPriority.Background);
        UpdateProgress(i, items.Count);
    }
}

}

Threading Rules

// Check if on UI thread if (!Dispatcher.CheckAccess()) { Dispatcher.Invoke(() => UpdateUI()); return; }

// From background thread await Task.Run(() => { var result = HeavyComputation();

// Marshal back to UI
Dispatcher.Invoke(() => DisplayResult(result));

});

Avoid

// ❌ Send priority - can cause deadlock Dispatcher.Invoke(() => {}, DispatcherPriority.Send);

// ❌ Blocking UI thread Thread.Sleep(1000);

// ✅ Use async/await instead await Task.Delay(1000);