title: WPF - error: [xxx] "does not contain a definition for [zzz] and no extension method [zzz] could be found
body: <p>I get an error while compiling:</p>
  
  <p>'SimulatorUi.MainWindow' does not contain a definition for 'UserCtrlSimulator' and no extension method 'UserCtrlSimulator' accepting a first argument of type 'SimulatorUi.MainWindow' could be found (are you missing a using directive or an assembly reference?) </p>
  
  <p>I got this error while compiling a user control and its owning window in the same project.</p>
  
  <p>Why ?</p>