Question

I have a DataGrid:

<DataGrid ...></DataGrid>

Somewhere there in the default template, there’s a ScrollViewer. I want to add content inside it. Currently I am doing this:

<Grid>
    <DataGrid Grid.Row="0">...</DataGrid>

    <Canvas Width="128" VerticalAlignment="Stretch" HorizontalAlignment="Left" Grid.Row="0" Margin="0,25,0,0">
        <Rectangle Width="32" Height="256" Canvas.Top="0" Canvas.Left="0" Stroke="Black" StrokeThickness="2" />
    </Canvas>
</Grid>

Which looks like this:

That is, the canvas gets drawn inside the grid. This is all great, but the canvas does not scroll along with the data grid. Now I realize I probably need to alter the control template and put the canvas inside the scroll viewer:

<DataGrid>
    <ControlTemplate>
        <ScrollViewer>
            <Grid>
                <ContentPresenter Grid.Row="0" />

                <Canvas Width="128" VerticalAlignment="Stretch" HorizontalAlignment="Left" Grid.Row="0" Margin="0,25,0,0">
                    <Rectangle Width="32" Height="256" Canvas.Top="0" Canvas.Left="0" Stroke="Black" StrokeThickness="2" />
                </Canvas>
            </Grid>
        </ScrollViewer>
    </ControlTemplate>
</DataGrid>

However, this is giving me the error:

A first chance exception of type ‘System.InvalidOperationException’
occurred in PresentationFramework.dll

Additional information: Items collection must be empty before using
ItemsSource.

How do I place the canvas inside the scroller to make it scroll along with the content?

Answer 1

DataGrid is an ItemsControl and the ContentProperty for an ItemsControl is the Items property. Here’s the decompiled source, notice the ContentProperty attribute:

[DefaultEvent("OnItemsChanged"), DefaultProperty("Items")]
[ContentProperty("Items")]
[StyleTypedProperty(Property = "ItemContainerStyle", StyleTargetType = typeof(FrameworkElement))]
[Localizability(LocalizationCategory.None, Readability = Readability.Unreadable)] // cannot be read & localized as string 
public class ItemsControl : Control, IAddChild, IGeneratorHost 
{

This means that if you do something like this:

<DataGrid>
    <SomethingHere>
</DataGrid>

Then <SomethingHere> – which is the content – is applied to the Items property. If you subsequently bind to the ItemsSource property you will get the error you’re seeing – “Items collection must be empty before using ItemsSource.”

If you had used

<DataGrid>
    <DataGrid.Template>
        <ControlTemplate> ...

You wouldn’t get the error.

In any case it would be better to extract the DataGrid’s full ControlTemplate using Blend or VS11 and modify that, e.g.

    <Style x:Key="{ComponentResourceKey ResourceId=DataGridSelectAllButtonStyle, TypeInTargetAssembly={x:Type DataGrid}}" TargetType="{x:Type Button}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type Button}">
                    <Grid>
                        <Rectangle x:Name="Border" Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}" SnapsToDevicePixels="True"/>
                        <Polygon x:Name="Arrow" Fill="Black" HorizontalAlignment="Right" Margin="8,8,3,3" Opacity="0.15" Points="0,10 10,10 10,0" Stretch="Uniform" VerticalAlignment="Bottom"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Setter Property="Stroke" TargetName="Border" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/>
                        </Trigger>
                        <Trigger Property="IsPressed" Value="True">
                            <Setter Property="Fill" TargetName="Border" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/>
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="False">
                            <Setter Property="Visibility" TargetName="Arrow" Value="Collapsed"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <Style x:Key="DataGridStyle1" TargetType="{x:Type DataGrid}">
        <Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
        <Setter Property="BorderBrush" Value="#FF688CAF"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="RowDetailsVisibilityMode" Value="VisibleWhenSelected"/>
        <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
        <Setter Property="ScrollViewer.PanningMode" Value="Both"/>
        <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type DataGrid}">
                    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" Padding="{TemplateBinding Padding}" SnapsToDevicePixels="True">
                        <ScrollViewer x:Name="DG_ScrollViewer" Focusable="false">
                            <ScrollViewer.Template>
                                <ControlTemplate TargetType="{x:Type ScrollViewer}">
                                    <Grid>
                                        <Grid.ColumnDefinitions>
                                            <ColumnDefinition Width="Auto"/>
                                            <ColumnDefinition Width="*"/>
                                            <ColumnDefinition Width="Auto"/>
                                        </Grid.ColumnDefinitions>
                                        <Grid.RowDefinitions>
                                            <RowDefinition Height="Auto"/>
                                            <RowDefinition Height="*"/>
                                            <RowDefinition Height="Auto"/>
                                        </Grid.RowDefinitions>
                                        <Button Command="{x:Static DataGrid.SelectAllCommand}" Focusable="false" Style="{DynamicResource {ComponentResourceKey ResourceId=DataGridSelectAllButtonStyle, TypeInTargetAssembly={x:Type DataGrid}}}" Visibility="{Binding HeadersVisibility, ConverterParameter={x:Static DataGridHeadersVisibility.All}, Converter={x:Static DataGrid.HeadersVisibilityConverter}, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}" Width="{Binding CellsPanelHorizontalOffset, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                        <DataGridColumnHeadersPresenter x:Name="PART_ColumnHeadersPresenter" Grid.Column="1" Visibility="{Binding HeadersVisibility, ConverterParameter={x:Static DataGridHeadersVisibility.Column}, Converter={x:Static DataGrid.HeadersVisibilityConverter}, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                        <ScrollContentPresenter x:Name="PART_ScrollContentPresenter" CanContentScroll="{TemplateBinding CanContentScroll}" Grid.ColumnSpan="2" Grid.Row="1"/>
                                        <ScrollBar x:Name="PART_VerticalScrollBar" Grid.Column="2" Maximum="{TemplateBinding ScrollableHeight}" Orientation="Vertical" Grid.Row="1" Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}" Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportHeight}"/>
                                        <Grid Grid.Column="1" Grid.Row="2">
                                            <Grid.ColumnDefinitions>
                                                <ColumnDefinition Width="{Binding NonFrozenColumnsViewportHorizontalOffset, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"/>
                                                <ColumnDefinition Width="*"/>
                                            </Grid.ColumnDefinitions>
                                            <ScrollBar x:Name="PART_HorizontalScrollBar" Grid.Column="1" Maximum="{TemplateBinding ScrollableWidth}" Orientation="Horizontal" Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}" Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}" ViewportSize="{TemplateBinding ViewportWidth}"/>
                                        </Grid>
                                    </Grid>
                                </ControlTemplate>
                            </ScrollViewer.Template>
                            <Grid>
                                <ItemsPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>

                                <!-- your canvas here -->
                                <Canvas Width="128" VerticalAlignment="Stretch" HorizontalAlignment="Left" Grid.Row="0" Margin="0,25,0,0">
                                    <Rectangle Width="32" Height="256" Canvas.Top="0" Canvas.Left="0" Stroke="Black" StrokeThickness="2" />
                                </Canvas>
                            </Grid>
                        </ScrollViewer>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <Trigger Property="IsGrouping" Value="true">
                <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
            </Trigger>
        </Style.Triggers>
    </Style>
</Window.Resources>
<Grid>
    <DataGrid HorizontalAlignment="Left" Margin="125,110,0,0" VerticalAlignment="Top" Height="134" Width="258" Style="{DynamicResource DataGridStyle1}"/>

</Grid>