ItemsControl not showing without DataContext binding

Question

I have window with the DataContext set to an instance of my view model. My view model has a property 'SelectedFile' of a custom class. That custom class has a property 'RevHistory' of type ObservableCollection where Revision is a custom class with a few string properties. RevHistory is initialized to a fixed size. My window has a part with an ItemsControl as follows (extra content removed)

<Window>
<Grid Grid.IsSharedSizeScope="True">
    ...
    <Border Grid.Column="1" Grid.Row="3" Grid.RowSpan="3" BorderBrush="{x:Static SystemColors.ControlDarkBrush}" BorderThickness="2" Margin="0,0,5,5" CornerRadius="3">
        <Grid>
        ...
            <ScrollViewer DataContext="{Binding SelectedFile, Mode=TwoWay}" Grid.Row="1" Margin="5" HorizontalAlignment="Stretch" VerticalScrollBarVisibility="Hidden" HorizontalScrollBarVisibility="Auto" VerticalAlignment="Top">
                <Grid>
                    ...
                    <Border Grid.Row="1" Grid.Column="0" BorderThickness="1,0,0,0" BorderBrush="{x:Static SystemColors.ActiveBorderBrush}">
                        <ItemsControl ItemsSource="{Binding RevHistory}">
                            <ItemsControl.ItemTemplate>
                                <DataTemplate>
                                    <TextBox BorderThickness="0,0,0,1" Text="{Binding Rev}" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" TextAlignment="Center" Padding="2"/>
                                </DataTemplate>
                            </ItemsControl.ItemTemplate>
                        </ItemsControl>
                    </Border>
                    ...
                </Grid>
            </ScrollViewer>
            ...
        </Grid>
    </Border>
</Grid>

What I'm having trouble understanding is why it only works when the DataContext binding is made at the ScrollViewer. Does the ScrollViewer block further DataContext inheritance? With the ItemsControl ItemSource={Binding SelectedFile.RevHistory} no items are shown. Going up the visual tree from the ItemsControl, the ScrollViewer was the first place that binding the DataContext worked.

why cant you set the DataContext of ItemsControl as SelectedFile? — Abin
If there is no other element in the hierarchy between the ScrollViewer and the ItemsControl where a DataContext is set, removing the DataContext assignment from the ScrollViewer and setting ItemSource="{Binding SelectedFile.RevHistory}" must work. There should not be any explicit DataContext assignment except the one at the top level, i.e. the window. Please try to provide a minimal reproducible example. — Clemens
@AbinMathew I couldn't get that to work. Working off of a single view model, I would normally just set the DataContext for the window as Clemens suggested. I thought it might be a notification issue, but the view model instance contains blank data before it's set as the DataContext, so it should have shown something. I'll work on posting a full example. — J Straight
@Clemens My "slim" version actually works as expected, so I went back to my original project. For the moment, removing Window.DataContext entry in the XAML is making it work. I thought it was only a type reference for helping with auto-complete in bindings, but I guess it's doing more than that. I'm using parameterized constructors in a few places, but making sure everything had a parameterless constructor didn't help. — J Straight

mZm mZm · Accepted Answer · 2019-12-15T10:29:06

That happens to me a lot. My guess the problem is when SelectedItem is null, then you get null reference exception that gets eaten by the control.

It should work if you move the DataContext={Binding SelectedItem} from ScrollViewer to the ItemsControl.

ItemsControl not showing without DataContext binding

1 Answers