Working with XmlPreloadedResolver
The XmlPreloadedResolver type is used as a resolver when automatic calls to the network are not desired or are not possible. For example, this type can be used for loading cached external DTDs. Or, you might use XmlPreloadedResolver to preload well-known DTDs that are defined in XHTML 1.0. Preloading well-known DTDs does not invoke any network connections, because these DTDs are already embedded in the System.Xml.Utils assembly. Currently, XmlPreloadedResolver includes DTDs defined in XHTML 1.0 and RSS 0.91.
In your Silverlight project, you must reference the .NET Framework assemblies that are installed with Silverlight, instead of those that are installed with Visual Studio.