Скачать или смотреть Solving the NullReferenceException in WPF Navigation within Windows

Discover how to navigate between pages in WPF while using a window, avoiding common pitfalls like `NullReferenceException`.
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This video is based on the question https://stackoverflow.com/q/70097546/ asked by the user 'Dankindonut' ( https://stackoverflow.com/u/14608498/ ) and on the answer https://stackoverflow.com/a/70097737/ provided by the user 'mm8' ( https://stackoverflow.com/u/7252182/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Navigating Between Pages in WPF: A Solution to NullReferenceException

When working with Windows Presentation Foundation (WPF), developers often encounter issues while trying to navigate between pages. A common problem arises when you attempt to use this.NavigationService.Navigate(...) in a page that is loaded within a frame of a window. If you've experienced this challenge, you're not alone—many developers face the same System.NullReferenceException error. In this guide, we will explore the problem in detail and provide a solution that allows for smooth navigation using windows.

Understanding the Problem

What Happens?

In WPF, when a page is embedded within a frame in a window, calling the NavigationService directly from that page can lead to a NullReferenceException. This happens because the NavigationService cannot locate a parenting Frame when that page is loaded within a Window.

A Common Scenario

Consider this example where we have a Frame named Main inside a MainWindow, and we navigate to Page1. When a button is clicked in Page1, you might try to navigate to another page (NextPage.xaml). However, the code below raises an exception:

[[See Video to Reveal this Text or Code Snippet]]

This code works when starting the application directly with Page1, but it fails when loaded inside a window.

Implementing the Solution

To navigate successfully between pages while avoiding errors, you need a way to access the parent window from the page loaded in the frame. Here’s how to fix this problem effectively:

Step-by-Step Guide

Retrieve the Parent Window: Use the Window.GetWindow(this) method to obtain a reference to the parent window. This method lets you access the window containing the page, allowing you to navigate without direct involvement of the NavigationService from the page itself.

Cast the Parent Window: Once you retrieve the parent window, you need to cast it to your specific MainWindow type. This ensures that you can access the Frame named Main.

Access and Navigate: Finally, navigate to the target page using the acquired frame's navigation service.

Here’s the Updated Code

Replace your button click handler with the following code:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Code:

Window.GetWindow(this): Obtains a reference to the parent window that contains the current page.

as MainWindow: Safely casts the window to your defined type (MainWindow).

parentWindow.Main.NavigationService.Navigate(...): Uses the navigation service of the Frame in the window to navigate to the new page.

Conclusion

By following this approach, you can resolve the NullReferenceException issue and navigate seamlessly between pages in a WPF application hosted within a window. This makes it much easier to build rich, user-friendly interfaces while maintaining control over your application's navigation.

If you're facing any additional concerns or queries about WPF navigation, feel free to leave a comment below!