Modifiying ObservableCollection from non-UI thread

[11v1]

Hi everyone, we are moving our application from WPF to Avalonia. In WPF, if you modify an ObservableCollection that has a binding by UI from not UI thread, you instantly get an exception. So you have 2 options:

1. Redirect modifications of ObservableCollection to the UI thread
2. Use external collection synchronization via BindingOperations.EnableCollectionSynchronization. This way, all modifications should be wrapped with lockObject by the developer. On the other side, WPF uses the same lockObject when it reads ObservableCollection.
   With Avalonia, we get no exceptions when we modify ObservableCollection outside of the UI thread. However, there's no BindingOperations.EnableCollectionSynchronization analog, so I assume the operations are not thread-safe. We catch the following exception sometimes on collection modification, which proves my concerns on thread-safety issues. Exceptions examples:

System.NullReferenceException: Arg_NullReferenceException
   at Avalonia.Utilities.WeakEvent`2.Subscription.OnEvent(Object sender, TEventArgs eventArgs)
   at Avalonia.Utilities.WeakEvents.<>c__DisplayClass5_0.<.cctor>b__5(Object _, NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   ...

or

System.NullReferenceException: Arg_NullReferenceException
   at Avalonia.Utilities.WeakHashList`1.GetAlive(Func`1 factory)
   at Avalonia.Utilities.WeakEvent`2.Subscription.OnEvent(Object sender, TEventArgs eventArgs)
   at Avalonia.Utilities.WeakEvents.<>c__DisplayClass5_0.<.cctor>b__5(Object _, NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   ...

My question is: Is there truly no analog of BindingOperations.EnableCollectionSynchronization in Avalonia, and we need to modify ObservableCollection on UI thread only? Or there's another way to achieve what we do in WPF (option 2)?

[timunie]

You can use Dispatcher.UIThread.Post to update the collection. The data can be fetched async.

Alternative is to use the package 3rd party ObservableCollections or DynamicData which is shipped via ReactiveUI

[11v1]

Hi @timunie, Dispatcher.UIThread.Post is easy, and I would have used it if I had received an exception from Avalonia that I should do it. However, I don't see it. I can modify the observable collection from any thread, and it usually works. This is my understanding why: when the binding happens, Avalonia needs to perform steps:

1. Enumerate all the existing items in the list
2. Subscribe to changes
3. Handle change events
   All bindings occur in the UI thread, which means step 1 always takes place on the UI thread. If some modification of the observable list happens during step 1 on some other thread (let's call it thread 2), the UI throws an exception and remains broken.
   My concern is that Avalonia is exposed to this kind of error. You can successfully modify the observable list on any thread until you get unlucky. It's a thread-race condition that we are not protected from. I assume most of the developers are not even aware of it. You can modify the collection from the UI thread (safe) or not the UI thread (not safe, thread-race condition), but Avalonia gives you to do this. I believe it shouldn't be like that. WPF is strict about it: either use the UI thread or use custom synchronization (BindingOperations.EnableCollectionSynchronization), otherwise you'll get an exception at runtime at the very same point you try to modify the collection.