The MainWindow control derives from UserControl and is meant to replace the default layout root. It provides the following features: A title bar that users can drag to move the window. The title bar consumes out-of-browser configuration data for the icon and title text. A default border that users can drag to resize the window. Hovering over the border changes the cursor as you would expect. A BorderBrush dependency property lets you customize the border background from the application layout root. Isolated Storage support to store the window size, position, and state on application shutdown and retrieve the values on the next startup.
The MainWindow control derives from UserControl and is meant to replace the default layout root. It provides the following features:
MEF has shipped with Silverlight 4 and with .NET Framework 4.0 and there are versions on CodePlex targeting Silverlight 3 and .NET Framework V3.5 Sp1.