of developing application user interfaces. The specific advantage that XAML brings to WPF is that XAML is a completely declarative language, allowing the developer (or designer) to describe the behavior and integration of components without the use of procedural programming. Although it is rare that an entire application will be built completely in XAML, the introduction of XAML allows application designers to more effectively contribute to the application development cycle. Using XAML to develop user interfaces also allows for separation of model and view, which is considered a good architectural principle. In XAML, elements and attributes map to classes and properties in the underlying APIs. As in web development, both layouts and specific themes are well suited to markup, but XAML is not required for either. Indeed, all elements of WPF may be coded in a .NET language (C#, VB.NET). The XAML code can ultimately be compiled into a managed assembly in the same way all .NET languages are. Architecture The architecture of WPF spans both managed code and native code components. However, the public API exposed is only available via managed code. While the majority of WPF is in managed code, the composition engine which renders the WPF applications is a native component. It is named Media Integration Layer (MIL) and resides in milcore.dll. It interfaces directly with DirectX and provides basic support for 2D and 3D surfaces, timer-controlled manipulation of contents of a surface with a view to exposing animation constructs at a higher level, and compositing the individual elements of a WPF application into a final 3D "scene" that represents the UI of the application and renders it to the screen. The Desktop Window Manager also uses the MIL for desktop and window composition. The media codecs are also implemented in unmanaged code, and are shipped as windowscodecs.dll. In the managed world, PresentationCore (presentationcore.dll) provides a managed wrapper for MIL and implements the core services for WPF, including a property system that is aware of the dependencies between the setters and consumers of the property, a message dispatching system by means of a Dispatcher object to implement a specialized event system and services which can implement a layout system such as measurement for UI elements. PresentationFramework (presentationframework.dll) implements the end-user presentational features, including layouts, time-dependent, story-board based animations, and data binding. WPF exposes a property system for objects which inherit from DependencyObject, that is aware of the