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, s

Answer 1

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;
    }
}

Answer 2

use a ToList() in order to iterate over the list.

foreach(var item in collection.ToList())
{
    if(item.Name == "Fred")
    {
        collection.Remove(item);
    }
}

Answer 3

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 4

if you really want to modify a collection you are going to want to iterate through a copy of the collection. its because you are trying to modify the collection in the foreach loop thats causing you grief.

example

var copy = new ObservableCollection<YourType>(collection)
foreach(var item in copy)
{
    if(item.Name == "Fred")
    {
        collection.Remove(item);
    }

}

that said, I agree with Anurag that you shouldn't be doing this type of thing with an observablecollection and certainly not inside a CollectionChanged Event.

Answer 5

Used this in the oncollectionchanged and it works (WPF and MVVM example):

new System.Threading.Thread(t =>
{
  Application.Current.Dispatcher.Invoke((Action)delegate
  {
    OnHoldMessages.Add(_selectedOnHoldMessage);
    RaisePropertyChanged(propertyName: "OnHoldMessages");
  });
}).Start();