Changing collection in CollectionChanged event results in exception

Description

If you change the collection within CollectionChanged event of an ObservableCollection an exception is thrown: Cannot change ObservableCollection during a CollectionChanged event.

Which at first makes sense and is probably by design to avoid ending in a loop it simply block us from making SAFE changes to the collection which dont end in a loop. Consider the following pseudo code:

var collection = new ObservableCollection<int>() { 0, 1, 2, 3, 4}

collection.CollectionChanged += (e, s) => {
	if (s.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add) {		
		collection.Move(ACollection[1], 0);
	}
};
collection.Add(5);

This code is absolutely safe to execute. But an exception is thrown. The only way to get it to work is moving the modification to another thread:

var collection = new ObservableCollection<int>() { 0, 1, 2, 3, 4}

collection.CollectionChanged += (e, s) => {
	if (s.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add) {		
		Bla();
	}
};
collection.Add(5);

async void Bla() {
	Application.Current.Dispatcher.Invoke(() => {	
		collection.Move(collection[1], 0);
	}, System.Windows.Threading.DispatcherPriority.Background);
}

I dont think changing it would be a breaking change because no existing code looks for and handle that specific exception. Instead of throwing an exception, a warning in VS analyzer might be better. Because an exception shouldn’t be used as a warning like it is now.

Reproduction Steps

See above

Expected behavior

No exception, at most a VS analyzer warning

Actual behavior

Exception is thrown

Regression?

No response

Known Workarounds

See above

Impact

No response

Configuration

.NET 6

Other information

No response

[Symbai]

I’ve created an issue in .NET runtime repo and linked it here. Closing this for now.

[lindexi]

@Symbai I find the code of ObservableCollection is in dotnet runtime. Should you move the issue to dotnet runtime?