WPF, Silverlight and Windows portable technologies all have their own implementation of a message box. In order to keep the ViewModels project technology-independent, any reference to a specific message box type is prohibited. If you want to show a message box from the ViewModels project, you can use the MessageBoxHelper.Show method. It is a service locator that needs to be initialized with the type you want to use for your message boxes. The method has one single parameter of type KeepItSoft.Fwk.Core.DialogEventArgs.