Here is a solution that uses (mostly) XAML. First, create a new ViewModel class that is a stub or mock. This class will also allow you to have some design-time data that won't crash Visual Studio. Then implement the following XAML to select between your main ViewModel and your mock ViewModel. The Blend XML Namespace ("d") is marked as Ignorable (only available during design-time) then AlternateContent is used to select the MockViewModel if the "d" namespace is available (design-time) and fallback to the MainViewModel (run-time).