New Post: How to use htmlagilitypack in Windows Phone 7

Like weiser said, just adding the System.Xml.XPath reference typically works!

As he said, it's typically located here: __C:\Program Files (x86)\Microsoft SDKs\Silverlight\v4.0\Libraries\Client __

However, it can cause problems. The only problem I ever found was the inability to navigate to a page inside a class library, using the /Namespace;component/Page.xaml format. This problem exists if...

• You have the System.Xml.XPath in the given class library
• You try to navigate to a page inside that class library.
  

Nevertheless, the fix is easy. Simply create a separate class library containing the page files, without any reference to the System.Xml.XPath. If your page requires XPath, then I suppose you'll be screwed. But in my case, my page was simply just a fullscreen photo viewer that was unrelated to my HTML parser/viewer.