Read original post by Diederik Krols at U2U Blog
This article describes how to create modal dialogs in a Windows 8 MVVM Metro-style app. Here's the use case: a main page opens a dialog and provides it with some information. The dialog is displayed in the center of the screen, and collects some information. As long as the dialog is open, the main page is inaccessible by the user interface. When the dialog is closed, the main page reads the result from the dialog, and continues processing.