WPF Bind to a different property path when binding value is null - wpf

Question

I need to do something similar to a PriorityBinding, except the lower priority binding is used when the binding is null (not when the binding is invalid like in a PriorityBinding). I can't seem to figure out a 'good' way to do this short of creating two duplicate controls one with each binding I want to use and triggering their visibility based on whether or not the binding is null. There must be a better way as I do not want to update two controls every time I need to change something (duplicate code == bad).
Example:
when SelectedItem.SelectedItem is not null:
<ContentControl Content="{Binding SelectedItem.SelectedItem}"/>
and when SelectedItem.SelectedItem is null:
<ContentControl Content="{Binding SelectedItem}"/>
using a Style like this did not work:
<ContentControl Content="{Binding SelectedItem.SelectedItem}">
<ContentControl.Style>
<Style TargetType="ContentControl">
<Style.Triggers>
<DataTrigger Binding="{Binding SelectedItem.SelectedItem}" Value="{x:Null}">
<Setter Property="Content" Value="{Binding SelectedItem}"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentControl.Style>
</ContentControl>
I'm guessing this did not work because the bindings in the style are trying to use the ContentControl's Content property as their source so the DataTrigger is acually testing SelectedItem.SelectedItem.SelectedItem.SelectedItem. Any ideas?

Answer 1

You could use MultiBinding to achive what you want:
<ContentControl Content="{Binding SelectedItem.SelectedItem}">
<ContentControl.Content>
<MultiBinding Converter="{StaticResource ResourceKey=myConverter}">
<Binding Path="SelectedItem"/>
<Binding Path="SelectedItem.SelectedItem"/>
</MultiBinding>
</ContentControl.Content>
</ContentControl>
Your Converter could look like
public class MyMultiConverter:IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (values[1] == null)
return values[0];
return values[1];
}
public object[] ConvertBack(object values, Type[] targetType, object parameter, CultureInfo culture)
{
return null;
}
}