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.

You cannot remove items from an ObservableCollection. That is the whole point of one... You would only be able to remove items from the collection that it is wrapping. — Daniel Lorenz, Mar 16 '12 at 19:36
You shouldn't do it in the collection changed event handler. Instead do the filter before you allow the collection to be changed. Dont allow the collection to be changed if its not an appropriate change. — Evan Larsen, Mar 16 '12 at 19:39
I know this is old and probably not the 'correct' way to do it; but it is what I did and may help someone. I was having the same issue of getting that exception when I was trying to delete selected files from a OberservableCollection. The CollectionChange event was firing still, when the second file removal request was being processed by my Messenger. So in my CollectionChanged event handler, I had to add `lock (this.Files) { // Code of the Event }` Once I did this, I ran my program again and selected multi files and hit delete. This time around no exception, since the Remove had to wait. — famousKaneis, Aug 25 '15 at 14:31

score 20 · Accepted Answer · edited Dec 09 '21 at 17:43

20

Check out Common Mistakes using Observable Collection.

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

edited Dec 09 '21 at 17:43

Guillaume Raymond

1,726
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answered Mar 16 '12 at 20:08

Raj Ranjhan

3,869
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2

Hi The link seems to be no longer available. – MetaSnarf Jan 02 '19 at 09:09
1

@MetaSnarf after some research I finally found a working link: https://ripcode.net/p/clinq/wiki/74840/adding-to-a-source-collection-from-a-collection-changed-event-handler – hybrid2102 Nov 22 '21 at 13:40

score 12 · Answer 2 · answered Mar 16 '12 at 20:24

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.

score 1 · Answer 3 · answered Dec 11 '18 at 08:14

1

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();

answered Dec 11 '18 at 08:14

Mark

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probably not the most optimal, but it works, thanks Mark. – ja73 Feb 20 '23 at 16:10

score 1 · Answer 4 · answered Jan 29 '21 at 20:00

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;
    }
}

score 0 · Answer 5 · answered Oct 30 '17 at 08:38

0

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

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

answered Oct 30 '17 at 08:38

Gilad

6,437
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Remove an item from an ObservableCollection in a CollectionChanged event handler

5 Answers5

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