TabStripPlacement binding exception in TabItem

[paulrozhkin]

• .NET Core Version: 6.0.100
• Windows version: Windows 10 Version 20H2 (OS Build 19042.1645)

• Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes

  Problem description: Binding failure occurs when removing last item (last tab) from TabControl. This is independent of whether MVVM or code behind is being used.

  Actual behavior: When deleting the last tab I get an error:

  Binding Path: TabStripPlacement;
  Target: TabItem; 
  Description: Cannot find source: RelativeSource FindAncestor, AncestorType='System.Windows.Controls.TabControl', AncestorLevel='1'

  Expected behavior: Binding exception does not occur

  Minimal repro: Delete tabs with buttons "Remove last" or "Remove all" MainWindow.xaml:

  <Window x:Class="TabControlBug.MainWindow"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      mc:Ignorable="d"
      Title="MainWindow" Height="450" Width="800">
  <Grid Margin="0,0,10,10">
      <Grid>
          <Grid.RowDefinitions>
              <RowDefinition />
              <RowDefinition Height="Auto" />
          </Grid.RowDefinitions>
  
          <TabControl x:Name="TabControl" Margin="10,10,0,0">
  
              <TabControl.ContentTemplate>
                  <DataTemplate>
                      <Grid>
                          <TextBox Text="{Binding Text}" FontSize="16" AcceptsReturn="True"
                                   HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />
                      </Grid>
                  </DataTemplate>
              </TabControl.ContentTemplate>
  
              <TabControl.ItemTemplate>
                  <DataTemplate>
                      <TextBlock Text="{Binding Header}" />
                  </DataTemplate>
              </TabControl.ItemTemplate>
          </TabControl>
  
          <StackPanel Grid.Row="1" Orientation="Horizontal">
              <Button Content="Add"
                      Margin="20 0 0 0"
                      Height="22" Click="Add_OnClick" />
              <Button Content="Remove last"
                      Margin="20 0 0 0"
                      Height="22" Click="RemoveLast_OnClick"/>
              <Button Content="Remove all"
                      Margin="20 0 0 0"
                      Height="22" Click="RemoveAll_OnClick" />
          </StackPanel>
  
      </Grid>
  </Grid>
  </Window>

  MainWindow.xaml.cs:

  
  using System;
  using System.Windows;

namespace TabControlBug { public partial class MainWindow { public MainWindow() { InitializeComponent(); AddItem(); AddItem(); AddItem(); }

    private void Add_OnClick(object sender, RoutedEventArgs e)
    {
        AddItem();
    }

    private void RemoveLast_OnClick(object sender, RoutedEventArgs e)
    {
        if (TabControl.Items.Count > 0)
        {
            TabControl.Items.RemoveAt(TabControl.Items.Count - 1);
        }
    }

    private void RemoveAll_OnClick(object sender, RoutedEventArgs e)
    {
        TabControl.Items.Clear();
    }

    private void AddItem()
    {
        var newItem = new TabItemViewModel
        {
            Header = Guid.NewGuid().ToString().Substring(0, 3),
            Text = Guid.NewGuid().ToString()
        };

        TabControl.Items.Add(newItem);
    }
}

}

TabItemViewModel.cs:

using System.ComponentModel; using System.Runtime.CompilerServices; using TabControlBug.Annotations;

namespace TabControlBug { public class TabItemViewModel : INotifyPropertyChanged { private string _text; private string _header;

    public string Header
    {
        get => _header;
        set
        {
            _header = value;
            OnPropertyChanged(nameof(Header));
        }
    }

    public string Text
    {
        get => _text;
        set
        {
            _text = value;
            OnPropertyChanged(nameof(Text));
        }
    }

    public event PropertyChangedEventHandler? PropertyChanged;

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

}

[to87al]

When using MVVM, this binding error becomes more evident. Check also Stackoverflow thread

[Torsteinws]

I found an old blogpost that describes a similar problem. The blogpost recommends to globally suppress all binding errors in the app. I did not like this because I only wanted to suppress binding errors that are managed internally by WPF.

Therefore, I ended up writing this workaround for my MVVM app:

Workaround in ViewModel class:

public void RemoveTabItem() 
{
  AppUtils.BindingErrors.Hide();

  // Remove TabItem here

  AppUtils.BindingErrors.Show();
}

Workaround:

namespace AppUtils;

using System.Windows;
using System.Diagnostics;
using System.Windows.Threading;

public static class BindingErrors
{

#if !DEBUG
    public static void Hide() { }
    public static void Show() { }
#endif

#if DEBUG
    private static SourceLevels _defaultLevel = PresentationTraceSources.DataBindingSource.Switch.Level;

    private static bool _isHiding = false;

    public static void Hide()
    {
        if (!_isHiding)
        {
            _isHiding = true;
            _defaultLevel = PresentationTraceSources.DataBindingSource.Switch.Level;
            PresentationTraceSources.DataBindingSource.Switch.Level = SourceLevels.Critical;
        }
    }

    public static void Show()
    {
        if (_isHiding)
        {
            /// Wait for UI to load before showing binding errors again
            Application.Current.Dispatcher.BeginInvoke(() =>
            {
                _isHiding = false;
                PresentationTraceSources.DataBindingSource.Switch.Level = _defaultLevel;
            }, DispatcherPriority.Loaded);
        }
    }
#endif
}