When should I use a UserControl instead of a Page?

Question

I notice that many of the WPF MVVM frameworks seem to avoid using the NavigationWindow and Page controls in favor of composing pages using nested UserControls.

The NavigationWindow and Page provide easy ways to enable back and forward navigation in the journal as well as providing an easy way to pass data among pages. Most MVVM frameworks I've seen re-implement these features in various ways.

Is there a specific reason to avoid using NavigationWindow and Page?

Answer 1

"NavigationWindow does not store an instance of a content object in navigation history. Instead, NavigationWindow creates a new instance of the content object each time it is navigated to by using navigation history. This behavior is designed to avoid excessive memory consumption when large numbers and large pieces of content are being navigated to. Consequently, the state of the content is not remembered from one navigation to the next. However, WPF provides several techniques by which you can store a piece of state for a piece of content in navigation history...."

http://msdn.microsoft.com/en-us/library/system.windows.navigation.navigationwindow.aspx

Answer 2

I just discovered another difference between UserControls and Pages: Pages cannot be used as DataTemplates.

For example, if you were creating application using the MVVM style, you might expect this to work:

    <DataTemplate DataType="{x:Type ViewModels:ProjectDashboardViewModel}">
        <Views:ProjectDashboardView />
    </DataTemplate>

But if the ProjectDashboardView is a Page, it will fail.