WPF Applying a trigger on binding failure

This question is a follow on from this one...

I am binding to a heterogeneous collection of objects, not all objects have the same set of properties. I am doing this in a datagrid. I would like to gray out the cell if the binding fails. Is there a way to apply a trigger if a binding fails?

EDIT: The answer below was suitable for my purposes, but i followed up with this question because I would 开发者_Python百科like to know how to do it (in a non hack fashion - i do love the hack however, don't get me wrong)

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As far as I know, you can't do this directly. However, if you can identify a value that will never be returned from successful bindings, you can create a DataTrigger whose binding has that value as its FallbackValue, and trigger on that same value:

<!-- Hibble returns only positive values -->
<DataTrigger Binding="{Binding Hibble, FallbackValue=-1}" Value="-1">
  <Setter Property="Background" Value="Red" />
</DataTrigger>

In theory it might be possible to omit the FallbackValue and trigger on {x:Static DependencyProperty.UnsetValue}, which would be much cleaner, but this doesn't appear to work in practice.