When to Use Dispatcher and When to Unsubscribe Events?

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When to Use Dispatcher and When to Unsubscribe Events?

水知 230

Hi community!

I'm a new developer and I have serveral questions about using the dispatcher and unsubscribing the events.

When should I use dispatcher? When I browsing the sample codes I never found them using dispatcher, but sometimes my app will crash if I don't use dispatcher in some places.

When should I unsubscribe the events? For a templated control, if it's a child control of other control, should I unsubscribe the events when the control is being disposed? If so, where should I dispose them? The finalizer or somewhere else? And how to unsubscribe the anonymous functions?

Thanks for help!

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Answer 1

Hi @水知 , thanks for reaching us on this platform!

When should I use dispatcher?

In WinUI 3, UI elements must only be accessed from the UI thread. If code runs on a background thread (Task.Run, async database calls...) and tries to touch the UI, you will get exceptions. This is when you use Dispatcher.

You update UI from code running on a background thread
Your async method resumes on a non-UI thread

When I browsing the sample codes I never found them using dispatcher

Most sample code simply runs on the UI thread or uses async/await that automatically returns to UI context.

but sometimes my app will crash if I don't use dispatcher in some places.

Your app crashes because after await, it sometimes runs on a background thread, and background thread are not allowed to touch the UI. So, the dispatcher is needed to move the code back to the UI thread before updating controls.

When should I unsubscribe events?

You should unsubscribe events whenever the subscriber (your control or page) may be garbage-collected but the publisher lives longer.

If the event source outlives the subscriber, unsubscribe them.

Controls that are children of ListView/GridView or templates must unsubscribe, because virtualization reuses and removes controls.

Where to unsubscribe:

Do NOT rely on finalizers (~ClassName) because WinUI controls often never run them reliably.
Do NOT rely on Dispose() as controls in WinUI do not implement Dispose.

Do unsubscribe in Unloaded or OnNavigatedFrom, depending on the control type.

Here is an example for templated child control generated by Copilot:

public MyChildControl()
{
    Loaded += OnLoaded;
    Unloaded += OnUnloaded;
}
private void OnLoaded(object sender, RoutedEventArgs e)
{
    SomeService.DataChanged += OnDataChanged;
}
private void OnUnloaded(object sender, RoutedEventArgs e)
{
    SomeService.DataChanged -= OnDataChanged;
}

For a templated control, if it’s a child control of another control, should I unsubscribe the events when the control is being disposed?

Yes, always unsubscribe.

"Being disposed" in WinUI is actually when Unloaded fires or the parent control removes the child.

Finalizers should never be used for event cleanup as they are too late and unreliable in WinUI.

Where to unsubscribe (mentioned above)

And how to unsubscribe the anonymous functions?

You cannot unsubscribe directly, because there is no reference to it.

The correct approach is storing the delegate in a variable and unsubscribe later.

Here is an example generated by Copilot:

private RoutedEventHandler _handler;
public MyControl()
{
    _handler = (s, e) => DoWork();
    Button.Click += _handler;
}
public void Cleanup()
{
    Button.Click -= _handler;
}

水知 230 Reputation points

2025-12-12T08:29:33.61+00:00

Thanks @Harry Vo (WICLOUD CORPORATION) !

If my control is used as a DataTemplate of a ListView, do I still need to unsubscribe the events? If so, how should I know when to remove? I always just simply set the ItemsSource of the ListView and I don't know if I need to change my way.

Here's what I usually do:

listView.ItemsSource = someSource; // After that listView.ItemsSource = newSource;
Harry Vo (WICLOUD CORPORATION) 3,820 Reputation points Microsoft External Staff Moderator

2025-12-12T08:42:42.56+00:00

Hi @水知 , you know it can be cleaned up automatic but let's just unsubscribe in any scenario, which is the safest and the best practice, no need to think if it required or not.

do I still need to unsubscribe the events?

So, yes, you still need to unsubscribe. Even though the control is “inside” a ListView via a DataTemplate:

ListView uses UI virtualization: it creates and reuses item containers for performance.

Controls in the DataTemplate may stay in memory after being removed from the visual tree if they are still subscribed to events on long-lived objects (ViewModels, services, static events, Dispatcher, timers, etc.).

If you don’t unsubscribe, each recycled item keeps references, which leads to memory leaks, unexpected behavior, crashes.

when to remove?

Use the Unloaded event of the control and fires whenever the item container is removed from the visual tree or if the control is recycled by virtualization. Do not rely on finalizers or Dispose(), because controls in XAML do not have reliable disposal.

I don't know if I need to change my way.

No, you can still set ItemsSource normally. The key is inside the control itself: make sure any subscriptions happen in Loaded and unsubscribe in Unloaded.

This is standard practice for ListView items with DataTemplates.
水知 230 Reputation points

2025-12-12T08:46:17.2566667+00:00

Thanks for help @Harry Vo (WICLOUD CORPORATION) !
Harry Vo (WICLOUD CORPORATION) 3,820 Reputation points Microsoft External Staff Moderator

2025-12-12T08:48:49.7233333+00:00

You're welcome! If you still have any question, please reach us on this platform! We willing to help you on any problems!

Answer 2

When to use a dispatcher typically depends on the context of your application, especially in UI programming. A dispatcher is used to manage the execution of tasks on a specific thread, often the UI thread. If you need to update UI elements from a background thread, you should use the dispatcher to ensure that these updates are performed on the UI thread. This prevents crashes and threading issues, which is likely why your app crashes in certain situations when you don't use it.

As for unsubscribing from events, it is essential to unsubscribe from events to prevent memory leaks. If your control is a child of another control, you should unsubscribe from its events when the control is being disposed. This is typically done in the Dispose method of the control, rather than in the finalizer. The finalizer is not the best place for this because it may not be called in a timely manner, and it can lead to resource management issues. Unsubscribing in the Dispose method ensures that the event handlers are removed while the control is still in a valid state.

References:

水知 230 Reputation points

2025-12-12T07:22:19.76+00:00

I don't think there's a dispose method for controls

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When to Use Dispatcher and When to Unsubscribe Events?

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