Comments by Triynko22 (Top 1 by date)

There is enough information. The "GlassEffect.IsEnabled" comes from the following project, for which the full source code is available:
http://www.codeproject.com/Articles/22sd820/Adding-Glass-Effect-to-WPF-using-Attached-Properti

When I downloaded that project and added the same attribute my WPF window, I got an identical error. The application loads fine, despite the error, and the glass effect is applied as expected. So I'm confused as to why the compiler designer is showing this error despite it compiling and running fine.