BlockUIContainer of Flow Document in WPF

BlockUIContainer: BlockUIContainer allows controls, video, bitmap, 2D graphics, or 3D graphics to be integrated into a document, all the noncontent elements (classes that derive from UIElement) is placed inside a document with the help of BlockUIContainer. BlockUIContainer is similar: it can host any UIElement, but it wraps it as a Block instead of an Inline. You can also change the property of BlockUIContainer according to your requirnment like: If you don't want the contained element to fill the whole width, you must use FrameworkElement layout settings.

You might wonder why you would ever want to place controls inside a document. After all, isn't the best rule of thumb to use layout containers for user-interactive portions of your interface, and flow layout for length, read-only blocks of content? However, in real-world applications there are many types of documents that need to provide some sort of user interaction (beyond what the Hyperlink content element provides).
Here's an example that places a button under a paragraph:

Example of BlockUIContainer:

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      HorizontalAlignment="Center" VerticalAlignment="Center"> 
      <FlowDocumentPageViewer>
        <FlowDocument>
            <Paragraph>
                To create an account click on Login Button.
            </Paragraph>
            <BlockUIContainer>
                <Button Content="Log In" />
            </BlockUIContainer>
         </FlowDocument>
      </FlowDocumentPageViewer>
</Page>

Output Window

Conclusion

Hope this article helps you to understand the working of BlockUIContainer of Flow Document in WPF.