Remove an item from an ObservableCollection in a CollectionChanged event handler

Question

I'm hoping to be able to reject some items after they have been added to an ObservableCollection. I am not able to subclass the ObservableCollection or use any sort of view, so I seem to be limited to using the one event handler defined (CollectionChanged) to perform a .Remove() on the prohibited items. It's fine if the items exist for the short period between the event being raised and then handled; the items should just not persist in the collection. Calling .Remove() within the CollectionChanged event handler doesn't appear to be allowed. At runtime .NET throws an InvalidOperationException:

"Cannot change ObservableCollection during a CollectionChanged event."

Personally I think .NET should allow me to. If I create an infinite loop, it's my own darn fault.

The code I would like to use would look like:

myCollection.CollectionChanged += (sender, args) =>
{
    if (args.Action == NotifyCollectionChangedAction.Remove)
        return;
    foreach (var itm in myCollection)
    {
        if (itm.name == "Fred")
            myCollection.Remove(itm);
    }
}

I'm not sure what options I have. Using a dispatcher doesn't seem to work. Triggering another event and placing the .Remove call in another handler is the only other option that comes to mind.

Answer 1

Check out Common Mistakes using Observable Collection.

Having said that, if you still want to go this route - you can spin a new Thread

Answer 2

I had tried using setting a flag to request collection add/remove changes and then when the System.Windows.Interop.ComponentDispatcher.ThreadIdle called my handler, doing my add or remove. That works. However, using a try-finally in the collection changed handler to unwire and then rewire that same collection changed event handler also bypassed the re-entrance issue:

private void MyDataGrid_CollectionChanged( object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e )
{
    try
    {
        dgRows.CollectionChanged -= MyDataGrid_CollectionChanged;
        
        switch( e.Action )
        {
            case NotifyCollectionChangedAction.Add:
                if( SomeTestIsTrue() )
                dgRows.Add( new vmRowObject() );
                break;
        }
    }
    finally
    {
        dgRows.CollectionChanged += MyDataGrid_CollectionChanged;
    }
}