My WPF app suddenly crashes: Exception Info: System.NotImplementedException

It is possible that it is a problem within WPF, but it is unlikely. Your code or a library you are using is almost certainly throwing the NotImplementedException - not WPF/XAML..

Check the inner exception and debug with First Chance Exceptions enabled to get the stack when the exception is first thrown. Intermittent exceptions like this are often race conditions on the timing or ordering of properties being set on WPF controls.

While I suspect this is application code related, A clean build (delete bin/obj directories) can rule out any corrupt baml files, and if you've upgraded framework versions recently, you might double check that was done fully.