Hide DataTrigger if RelativeSource doesn't exist - wpf

Question

I want to add a DataTrigger to my base TextBox style so that it sets the foreground color to a different value if it is inside of a DataGridCell that is selected. Here is what my trigger looks like:
<Style.Triggers>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGridCell}}, Path=IsSelected}"
Value="True">
<Setter Property="Foreground"
Value="White" />
</DataTrigger>
</Style.Triggers>
This works great, except that when my TextBox is not in a DataGrid the Binding fails and writes an exception to the output window. How can I prevent this.
I basically want to say if Parent is a DataGridCell then apply this trigger otherwise ignore it.

Answer 1

In general just only apply the style where applicable. If you want implicit application use nested styles:
<Style TargetType="{x:Type DataGrid}">
<Style.Resources>
<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<DataTrigger
Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGridCell}}, Path=IsSelected}"
Value="True">
<Setter Property="Foreground" Value="White" />
</DataTrigger>
</Style.Triggers>
</Style>
</Style.Resources>
</Style>
If you have other parts which you want to apply to all TextBoxes take out those parts in a serarate style and use BasedOn in the style which applies to the TextBoxes inside the DataGrid.
Edit: MultiDataTrigger seems to return right away if a condition is not met so you can avoid binding errors:
<Style TargetType="{x:Type TextBox}">
<Style.Resources>
<vc:HasAncestorOfTypeConverter x:Key="HasAncestorOfTypeConverter" AncestorType="{x:Type DataGridCell}" />
</Style.Resources>
<Style.Triggers>
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition
Binding="{Binding RelativeSource={RelativeSource Self}, Converter={StaticResource HasAncestorOfTypeConverter}}"
Value="True" />
<Condition
Binding="{Binding RelativeSource={RelativeSource AncestorType={x:Type DataGridCell}}, Path=IsSelected}"
Value="True" />
</MultiDataTrigger.Conditions>
<Setter Property="Foreground" Value="Red" />
</MultiDataTrigger>
</Style.Triggers>
</Style>
public class HasAncestorOfTypeConverter : IValueConverter
{
public Type AncestorType { get; set; }
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (value == null) return false;
DependencyObject current = value as DependencyObject;
while (true)
{
current = VisualTreeHelper.GetParent(current);
if (current == null)
{
return false;
}
if (current.GetType() == AncestorType)
{
return true;
}
}
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotSupportedException();
}
}
This of course causes quite som overhead so it might not be such a good solution, then again if the RelativeSource-binding fails it also had to go up the tree first.