Question

The goal here would be to check all grid checkboxes if the header checkbox changes:

<Window.Resources>

    <Style TargetType="CheckBox" x:Key="InnerBox">
        <Setter Property="HorizontalAlignment" Value="Center" />
        <Style.Triggers>
            <DataTrigger Value="True"
                         Binding="{Binding IsChecked, 
                         ElementName=HeaderCheckbox}">
                <Setter Property="IsChecked" Value="True" />
            </DataTrigger>
            <DataTrigger Value="False"
                         Binding="{Binding IsChecked, 
                         ElementName=HeaderCheckbox}">
                <Setter Property="IsChecked" Value="False" />
            </DataTrigger>
        </Style.Triggers>
    </Style>

</Window.Resources>

<DataGrid>
    <DataGrid.Columns>

        <!-- col1 -->
        <DataGridTemplateColumn>
            <DataGridTemplateColumn.HeaderTemplate>
                <DataTemplate>
                    <!-- header check -->
                    <CheckBox Name="HeaderCheckbox" />
                </DataTemplate>
            </DataGridTemplateColumn.HeaderTemplate>
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <!-- body check -->
                    <CheckBox Style="{StaticResource InnerBox}" />
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>

        <!-- col2 -->
        <DataGridTextColumn Binding="{Binding}" Header="Text" />
    </DataGrid.Columns>

    <!-- sample data -->
    <sys:String>1</sys:String>
    <sys:String>2</sys:String>
    <sys:String>3</sys:String>
</DataGrid>

Looks like:

For some reason, the trigger does not fire.

Any ideas?

Answer 1

ElementName binding inside a DataTemplate can’t reach an element outside of the template as you noticed. This is because it can be instantiated many times and has its own namescope so any ElementName binding you create inside a DataTemplate will look inside the template for another element with that name.

Looking at it with Snoop we can also see that a RelativeSource binding can’t be used directly since they are in different parts of the Visual Tree

The only thing that I can think of to get around this is to bind both of the CheckBoxes to a common ancestor, e.g. the parent DataGrid and use an attached property or the Tag property. Example

<Style TargetType="CheckBox" x:Key="InnerBox">
    <Setter Property="HorizontalAlignment" Value="Center" />
    <Setter Property="IsChecked" Value="False" />
    <Style.Triggers>
        <DataTrigger Value="True"
                     Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGrid}},
                                       Path=Tag}">
            <Setter Property="IsChecked" Value="True" />
        </DataTrigger>
    </Style.Triggers>
</Style>

and

<DataTemplate>
    <!-- header check -->
    <CheckBox Name="HeaderCheckbox"
              IsChecked="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGrid}},
                                  Path=Tag,
                                  Mode=OneWayToSource}"/>
</DataTemplate>