WPF Dispatcher Threading

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);