Custom Markup Extensions
Custom markup extensions have been part of the WPF engine for some time. They are a welcome addition to Silverlight. Custom markup extensions allow the developer to create special tags within XAML that can perform any type of processing needed as the XAML tree is parsed. The extension is passed a context that provides access to the underlying objects and properties in which it is being used. This provides tremendous flexibility for building special tools and utilities that can both enhance the effectiveness of XAML as well as improve the readability. Specifically extensions can provide a cleaner localization syntax as well as declarative binding of events to methods in MVVM applications.