Question

I have the following template for some header:

<telerik:RadExpander.Header>
    <StackPanel Orientation="Horizontal" DataContext="{Binding CurrentItem, ElementName=activityProductGrid}">
        <TextBlock Text="{Binding Strings[Product], Source={StaticResource LanguageResources}, StringFormat='{}{0}: '}" />
        <TextBlock Text="{Binding Product.Name}" FontWeight="Bold" />
        <TextBlock Text="{Binding Strings[GroupName], Source={StaticResource LanguageResources}, StringFormat=', {0}: '}" />
        <TextBlock Text="{Binding GroupName}" />
        <TextBlock Text="{Binding Strings[UnitPrice], Source={StaticResource LanguageResources}, StringFormat=', {0}: '}" />
        <TextBlock Text="{Binding UnitPrice}" />
    </StackPanel>
</telerik:RadExpander.Header>

How can I do conditional formmatting of the header, if for example Product == null, then Header should not display anything?

Edit: This one works.

<StackPanel Orientation="Horizontal" DataContext="{Binding CurrentItem, ElementName=activityProductGrid}">
    <i:Interaction.Triggers>
        <ie:DataTrigger Binding="{Binding}" Value="{x:Null}">
            <ie:ChangePropertyAction TargetObject="{RelativeSource={RelativeSource AncestorType=StackPanel}}" PropertyName="Visibility">
                <ie:ChangePropertyAction.Value>
                    <Visibility>Collapsed</Visibility>
                </ie:ChangePropertyAction.Value>
            </ie:ChangePropertyAction>
        </ie:DataTrigger>
    </i:Interaction.Triggers>

    <-- from this point the came code as above -->
</StackPanel>

Answer 1

Specifically for null, you can specify binding parameters for a replacement value, or one to display in case of binding errors:

<TextBlock 
  Text="{Binding Product.Name, TargetNullValue=(empty), FallbackValue=(error)}"/>

(Taken from the WPF Binding Cheatsheet)

EDIT: I noticed that you didn’t just want a specific binding to have a different value, but the whole control to not be displayed. For this, you can use Styles and Triggers, binging a DataTrigger to your Product property, and setting Visibility if it’s null.

<telerik:RadExpander.Header>
    <StackPanel>
            <StackPanel.Style>
              <Style TargetType="StackPanel">
                <Style.Triggers>
                  <DataTrigger Binding="{Binding Product}" Value="{x:Null}">
                    <Setter Property="Visibility" Value="Collapsed"/>
                  </DataTrigger>
                </Style.Triggers>
              </Style>
            </StackPanel.Style>
        </StackPanel>
</telerik:RadExpander.Header>