SelectedItem Issue

[one8loop]

Hello everyone,
I'm facing an issue in my application, where I basically have a DataGrid with a binding to a DataGridCollectionView initialized from an ObservableCollection. I need to manage the SelectedItem (which represents a whole row) so that it is setted both by the View (user selection) and programmatically in the ViewModel (TwoWay Binding).

I face a random issue when I try to set the SelectedItem property from the ViewModel.
The problem is that sometimes a change from null to a valid reference to an object of the DataGridCollectionView fails, and I see two calls to a OnSelectedItemChanged callback in my ViewModel.

1. ViewModel::OnSelectedItemChanged: old value = null -> new value = <object>
2. ViewModel::OnSelectedItemChanged: old value = <object> -> new value = null

I digged into the DataGrid.cs code and found the the problem lies in the OnSelectedItemChanged method of DataGrid, specifically at the end when this check is performed:

// DataGrid.cs
private void OnSelectedItemChanged(AvaloniaPropertyChangedEventArgs e)
{
	[...]
	if (!_successfullyUpdatedSelection)
	{
		SetValueNoCallback(SelectedIndexProperty, selectedIndex);
		SetValueNoCallback(SelectedItemProperty, e.OldValue);
	}
}

When I encounter the wrong behaviour, _successfullyUpdatedSelection is false.
The method is reverting all changes and is setting SelectedItemProperty to the old value, which is null.
This is the source of the second event described above, the callback to OnSelectedItemChanged() (in the ViewModel) with null as the new value.

I've investigated further in the call stack of DataGrid's OnSelectedItemChanged, to understand why _successfullyUpdatedSelection remains set to false and I've concluded that the culprit lies in the following lines:

// DataGrid.cs
internal bool UpdateSelectionAndCurrency(int columnIndex, int slot, DataGridSelectionAction action, bool scrollIntoView)
{
	_successfullyUpdatedSelection = false;
	[...]
	try
	{
		[...]
		if (DataConnection.CollectionView != null && DataConnection.CollectionView.CurrentPosition != rowIndex)
		{
			DataConnection.MoveCurrentTo(item, slot, columnIndex, action, scrollIntoView);
		}
		else
		{
			ProcessSelectionAndCurrency(columnIndex, item, slot, action, scrollIntoView);
		}
	}
	[...]
	return _successfullyUpdatedSelection;
}

The method ProcessSelectionAndCurrency() is actually responsible for setting _successfullyUpdatedSelection = true but it is not called in each code path. If DataConnection.CollectionView.CurrentPosition != rowIndex then just DataConnection.MoveCurrentTo() is called, and it does not process the Selection at all.

That's what I found in my debug session.

I kindly ask DataGrid experts if there's something actually wrong in the code spotted above.
My opinion is that:

• ProcessSelectionAndCurrency() should be always called, or
• _successfullyUpdatedSelection should be set to true when the DataConnection.MoveCurrentTo() path is taken.

Thanks!