BindableCollection races against any call which adds/removes elements before it

[canton7]

Discussed in #306

^{Originally posted by Yoooi0 November 21, 2021}
There seems to be an issue with thread safety in BindableCollection.

When two threads modify the collection at the same time, one calling ClearItems, the other calling RemoveItem, it can cause OnCollectionChanging to throw an exception.

System.Reflection.TargetInvocationException: "An error occurred while dispatching a call to the UI Thread"

Inner Exception
ArgumentOutOfRangeException: "Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')"

at Stylet.Execute.OnUIThreadSync(Action action)
at Stylet.BindableCollection`1.RemoveItem(Int32 index)
at System.Collections.ObjectModel.Collection`1.Remove(T item)

Example timeline:

• Create BindableCollection with 1 item
• Thread 1 calls collection.ClearItems()
• Thread 1 enters Execute.OnUIThreadSync
• Thread 2 calls collection.Remove(firstItem), this calls RemoveItem with index argument set to 0
• Thread 2 waits for thread 1
• Thread 1 clears the collection by calling base.ClearItems()
• Thread 2 enters Execute.OnUIThreadSync
• Thread 2 throws exception when calling OnCollectionChanging because the item at index 0 does not exist anymore