Reputation: 1290

An ItemsControl is inconsistent with its items source - WPF Listbox

I have a WPF window containing a ListBox control that is populated when a button click method is executed.

XAML:

<ListBox Name="ThirdPartyListBox" ItemsSource="{Binding}" Margin="0,70,0,0">                      
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Orientation="Horizontal">
                                <Image Source="C:\Users\Test\Desktop\Project\ACME-WPF\ACME-WPF\window-new-3.ico" Margin="5" Width="50"/>
                                <Button Name="ThirdPartyInstallButton" Content="Install" Click="InstallThirdPartyUpdatesButton_Click" Margin="5,5,0,0" Height="25"></Button>
                                <Button Name="ThirdPartyPostoneButton" Content="Postpone" Click ="PostponeThirdPartyUpdatesButton_Click" Margin="5,5,0,0" Height="25"></Button>
                                <TextBlock FontWeight="Bold" Text="{Binding Item2.Name}" Margin="12,25,0,0"/>
                                <TextBlock FontWeight="Bold" Text="{Binding Item2.RequiredVersion}" Margin="3,25,0,0"/>
                                <TextBlock Text="{Binding Item2.CustomUIMessage}" Margin="10,25,0,0" TextWrapping="Wrap" Foreground="Red"/>
                                <TextBlock Text="You have used " Margin="3,25,0,0"/>
                                <TextBlock Text="{Binding Item3.UsedDeferrals}" Margin="3,25,0,0"/>
                                <TextBlock Text=" of " Margin="3,25,0,0"/>
                                <TextBlock Text="{Binding Item2.MaxDefferals}" Margin="3,25,0,0"/>
                                <TextBlock Text=" deferrals for this update." Margin="3,25,0,0"/>
                            </StackPanel>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>

C#:

 private void CheckforThirdPartyUpdatesButton_Click(object sender, RoutedEventArgs e)
    {
        CheckforThirdPartyUpdatesButton.IsEnabled = false;

        worker = new BackgroundWorker();
        worker.WorkerReportsProgress = true;
        worker.WorkerSupportsCancellation = true;

        worker.DoWork += delegate(object s, DoWorkEventArgs args)
        {
            MainEntry.checkFor3PUpdates();
        };

        worker.ProgressChanged += delegate(object s, ProgressChangedEventArgs args)
        {

        };

        worker.RunWorkerCompleted += delegate(object s, RunWorkerCompletedEventArgs args)
        {

            ThirdPartyListBox.DataContext = RegScan_ThirdParty.comparisonListWithState;
            CheckforThirdPartyUpdatesButton.IsEnabled = true;
        };

        worker.RunWorkerAsync();
    }

Everything up to this point functions as expected and the listbox is populated with multiple rows of items depending on how many items are in list ThirdPartyListBox.DataContext = RegScan_ThirdParty.comparisonListWithState;. However, if I interact with the listbox items at all, an InvalidOperationException is thrown with inner exception "An ItemsControl is inconsistent with its items source."

Can someone help me understand what's happening?

Upvotes: 28

Answers (6)

Bolek

Reputation: 81

I had the same error "An ItemsControl is inconsistent with its items source" but was doing everything from the UI thread. I was using a class implementing ICollectionView as the ItemsSource. I developed the view myself.

Turns the problem was the view did not invalidate enumerators it returned from ICollectionView.GetEnumerator() prior to raising the CollectionChanged event. ItemsSource internally caches enumerators and whatever results it got by using them. If the enumerators are not invalidated, it will happily reuse them and the derived values, including item count.

To invalidate enumerator, you throw InvalidOperationException from its Current property and MoveNext() and Reset() methods. ItemsControl will then request a new enumerator and recreate the cached data. If your enumerator implements IDisposable, it will also call Dispose() on it first.

Upvotes: 0

John Melville

Reputation: 3825

If your collection implements an INotifyCollectionChanged you can also sent a NotifyCollectionChangedAction.Reset message to basically say "I messed everything up, lets just start over."

Upvotes: -1

SErnst

Reputation: 141

I had the same error. It was caused by editing objects in the List, used as ItemsSource.

My solution was to replace the List with an ObservableCollection. According to .NET documentation an ObservableCollection:

"Represents a dynamic data collection that provides notifications when items get added, removed, or when the whole list is refreshed."

Upvotes: 10

neptunao

Reputation: 607

Such exceptions are thrown when an item's source has changed from another thread and ListBox doesn't receive a notification (CollectionChanged event) about ItemsSource being changed; so when it starts to do some work (like updating layout) it will see that Items are not equal to ItemsSource and throws an exception.

Since .NET 4.5, WPF provides a way to enable synchronization with a collection that changes from different threads. Try to use the EnableCollectionSynchronization method on your ItemsSource. See an example usage in this answer to a similar question

Upvotes: 33