I had this control that functioned as a textbox. The request was that it behaved differently when used as an editor in a grid and was in the "add new record" line. I agree that the best solution would have been for the grid to set a property to the editor when placing it somewhere or at least when adding to the grid, but that is not relevant.
The solution was, of course, to add a DataTrigger that would check a property of the parent grid row object, by using RelativeSource and FindAncestor. However, that approach resulted in an ugly binding error in the Visual Studio debug view window when the textbox was being used stand alone. It didn't break execution, but I kind of disliked it.
The solution, at first, was to use a Converter. However, the converter class does not apply to the RelativeSource, but to the referenced property. The binding would first look for the parent row, find nothing, then return a null reference exception when trying to access the path property.
Another solution would be to look for the parent in the LayoutUpdated event (only the first time) then look for the trigger and enable or disable it. Yet another is to reference a property that the textbox would set depending on the parent found above. Both solutions would be cumbersome if you don't have ownership of either control.
It appears that there is no way to determine the way a relative source is found. I don't know if they changed that in the WPF4 implementation, but it would have been a nice touch. That being said, I believe one can create a custom Binding object to take care of this. I will update the post with some sources when I get to working with WPF again.
Update: I've reached the conclusion that the problem is the question, not the answer. In order to make an element have a certain behaviour when a child of another element, a style should be added to the resources of the parent. This way, the problem is solved. I agree that sometimes it is cumbersome to do so, but the alternative is that your control will always search for a parent that might not be there. So, the solution is to make the parent make the child do what you want.