Bind to DependencyProperty of Window

I want to create a CheckBox for toggling the Topmost property of the window.

If TopMost was a property of the ViewModel, I’d use this:

<CheckBox IsChecked="{Binding Path=Topmost}">Topmost</CheckBox>

But Topmost is a DependencyProperty on the Window in which the CheckBox is situated.
I tried the following:

<CheckBox IsChecked="{Binding Path=Topmost, Source={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}">Topmost</CheckBox>

but I get the following error at runtime:

System.Windows.Data Error: 40 : BindingExpression path error: 'Topmost' property not found on 'object' ''RelativeSource' (HashCode=12083439)'. BindingExpression:Path=Topmost; DataItem='RelativeSource' (HashCode=12083439); target element is 'CheckBox' (Name=''); target property is 'IsChecked' (type 'Nullable`1')

What’s my mistake here?

I could give the window an x:Name="MyWindow" and then use

<CheckBox IsChecked="{Binding Path=Topmost, ElementName=MyWindow}">Topmost</CheckBox>

instead, but I’d prefer to avoid x:Name if possible.

>Solution :

You have to set the RelativeSource property of the Binding instead of the Source property:

<CheckBox Content="Topmost"
          IsChecked="{Binding Topmost,
                      RelativeSource={RelativeSource AncestorType=Window}}"/>