ConcurrentObservableCollection -> Remove Item

[Nowohier]

Hi! I´m having an issue using the ConcurrentObservableCollection, C#, WPF. Simple Setup:

In XAML I´m using a ComboBox within a UserControl. The ComboBox is bound to a ConcurrentObservableCollection using AsObservable and is also bound to the SelectedItem.

When trying to remove the SelectedItem from within the ViewModel like this:

if (TransferLists.Contains(SelectedTransferList)) TransferLists.Remove(SelectedTransferList); //TransferLists.Contains(SelectedTransferList)) returns "true"

I get the following Error (NullReferenceException):

bei Meziantou.Framework.WPF.Collections.ConcurrentObservableCollection1.System.Collections.IList.Contains(Object value) in /_/src/Meziantou.Framework.WPF/Collections/ConcurrentObservableCollection.cs: Zeile275 bei Meziantou.Framework.WPF.Collections.DispatchedObservableCollection1.System.Collections.IList.Contains(Object value) in /_/src/Meziantou.Framework.WPF/Collections/Internals/DispatchedObservableCollection.cs: Zeile345
bei System.Windows.Data.ListCollectionView.InternalContains(Object item)
bei System.Windows.Data.ListCollectionView.Contains(Object item)
bei System.Windows.Controls.ItemCollection.Contains(Object containItem)
bei System.Windows.Controls.Primitives.Selector.SelectionChanger.Select(ItemInfo info, Boolean assumeInItemsCollection)
bei System.Windows.Controls.Primitives.Selector.SelectionChanger.SelectJustThisItem(ItemInfo info, Boolean assumeInItemsCollection)
bei System.Windows.Controls.Primitives.Selector.OnSelectedItemChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
bei System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
bei System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
bei System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
bei System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
bei System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp, Boolean preserveCurrentValue)
bei System.Windows.Data.BindingExpressionBase.Invalidate(Boolean isASubPropertyChange)
bei System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
bei System.Windows.Data.BindingExpression.ScheduleTransfer(Boolean isASubPropertyChange)
bei MS.Internal.Data.ClrBindingWorker.NewValueAvailable(Boolean dependencySourcesChanged, Boolean initialValue, Boolean isASubPropertyChange)
bei MS.Internal.Data.PropertyPathWorker.UpdateSourceValueState(Int32 k, ICollectionView collectionView, Object newValue, Boolean isASubPropertyChange)
bei MS.Internal.Data.ClrBindingWorker.OnSourcePropertyChanged(Object o, String propName)
bei MS.Internal.Data.PropertyPathWorker.OnPropertyChanged(Object sender, PropertyChangedEventArgs e)
bei System.Windows.WeakEventManager.ListenerList`1.DeliverEvent(Object sender, EventArgs e, Type managerType)
bei System.ComponentModel.PropertyChangedEventManager.OnPropertyChanged(Object sender, PropertyChangedEventArgs args)
bei AGT.Main.ViewModel.BaseViewModel.OnPropertyChanged(String propertyName) in C:\CODE_COPY\Agt\Solace_MA\AGT.Main\ViewModel\BaseViewModel.cs: Zeile19

The Error is thrown in my BaseViewModel in the OnPropertyChanged Method:

` public class BaseViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;

    public virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    public virtual bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = "")
    {
        if (EqualityComparer<T>.Default.Equals(storage, value)) return false;
        storage = value;
        OnPropertyChanged(propertyName);
        CommandManager.InvalidateRequerySuggested();
        return true;
    }
}`

Do you have an idea how to overcome this error or what I´m doing wrong?

Thanks a lot in advance!

Chris

[Nowohier]

The error is not triggered by the remove method itself. When removing the item, also the setter of SelectedTransferList property is triggered by wpf user control causing a call of the SetProperty method in the BaseViewModel. This is when the Exception is eventually thrown. If I remove an item that is not selected, therefore not beeing displayed and not triggering SetProperty, all is fine.

[Nowohier]

Is it maybe some kind of race condition, because the item is not removed at once in ConcurrentObservableCollection, but is enqueued? Same code with a normal ObservableCollection runs without an issue. All methods used for this example are synchronuous.

[meziantou]

If you look at the Contains method, the only part of the method that can throw a NullReferenceException in some specific cases is the explicit cast:

        // https://github.com/meziantou/Meziantou.Framework/blob/1d2c6f6b8f96d161f4675fe55ae26427a5c651d6/src/Meziantou.Framework.WPF/Collections/ConcurrentObservableCollection.cs#L275
        bool IList.Contains(object? value)
        {
            AssertType(value, nameof(value));
            return Contains((T)value!); // The cast here (line 275)
        }

• What are the types of TransferLists and SelectedTransferList?
• Do you have a small repro, or at least some lines of code to explain the usage?
• Which version of .NET do you use? (net48, netcoreapp3.1, net5, ...)