page.DataContext not inherited from parent Frame?

Question

I have a Page page in a Frame frame, with frame.DataContext = "foo".

• (page.Parent as Frame).DataContext is "foo". ok
• BindingExpression for page.DataContext is null (also forced with ClearValue). ok
• page.DataContext is null. but I expected "foo"!

Why isn't the DataContext inherited? As far as I understand the Frame sandboxes the content. But I couldn't find any documentation of this behavior - can anyone point me to a place where this is mentioned?

Answer 1

To answer your question about documentation of this behavior: It's not Microsoft documentation, but I have a couple of WPF books that both mention this.

"Essential Windows Presentation Foundation" says: (pp. 160-161)

There are two interesting models for hosting navigable content: isolated hosting and integrated hosting.

With isolated hosting the content is not trusted and is run in a completely isolated (sandboxed) environment. This is how WPF content is hosted when running in the system Web browser as a XAML Browser Application. For navigation to another application or HTML content, this isolated hosting model is supported with a Frame object.

Integrated hosting, in which we want the content to behave as part of our application, is not supported at all in the system. When Frame navigates to content within the application, we get an odd hybrid of isolated and integrated behavior. Frame isolates its content from its style (and its parent's style), but not from the application's style. Events don't bubble from the content in Frame; however, the objects are accessible from the Content property (meaning that they aren't isolated in a security sense).

For all these reasons, Frame is most useful when we're working with external content, but it can be carefully used for application content.

That's all it has to say -- nothing about property inheritance.

"Windows Presentation Foundation Unleashed says (p. 95):

The Frame control holds arbitrary content, just like all other content controls, but it isolates the content from the rest of the UI. For example, properties that would normally be inherited down the element tree stop when they reach the Frame.

Answer 2

You didn't specifically ask how you could make this work, only why it doesn't by default. However, if you do want your Pages to inherit the Frame's DataContext, you can do this:

In XAML:

<Frame Name="frame"
       LoadCompleted="frame_LoadCompleted"
       DataContextChanged="frame_DataContextChanged"/>

In codebehind:

private void frame_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
    UpdateFrameDataContext();
}
private void frame_LoadCompleted(object sender, NavigationEventArgs e)
{
    UpdateFrameDataContext();
}
private void UpdateFrameDataContext(object sender, NavigationEventArgs e)
{
    var content = frame.Content as FrameworkElement;
    if (content == null)
        return;
    content.DataContext = frame.DataContext;
}