title: Prevent to close parent window if child windows are active
body: <p>I am developing multiple instance WPF application.  Application has a grid in a main screen and on double click of grid row, it opens child window. It has also functionality to open multiple child windows on double click of grid row from main screen. </p>
  
  <p>Can anybody help me to prevent parent window to be close if child windows are active? So user can not able to close main window if child windows are active.</p>