WPF Architecture

For every new technology, it is very essential to have a clear idea about its architecture. So before beginning your application, you must grab a few concepts. If you would not like to know WPF in detail, please skip this section. As mentioned earlier, WPF is actually a set of assemblies that build up the entire framework. These assemblies can be categorized as:
Managed Layer
UnManaged Layer
Core API

Managed Layer: Managed layer of WPF is built using a number of assemblies. These assemblies build up the WPF framework, communicate with lower level unmanaged API to renderits  content. The few assemblies that comprise the WPF framework are:

PresentationFramework.dll: Creates the top level elements like layout panels, controls, windows, styles, etc.

PresentationCore.dll: It holds base types such as UIElement, Visual from which all shapes and controls are Derived in PresentationFramework.dll.

WindowsBase.dll: They hold even more basic elements which are capable of being used outside the WPF environment like Dispatcher object, Dependency Objects. I will discuss each of them later.

Unmanaged Layer (milcore.dll): The unmanaged layer of WPF is called milcore or Media Integration Library Core. It basically translates the WPF higher level objects like layoutpanels,buttons, animation, etc. into textures thatDirect3D expects. It is the main rendering engine of WPF.

WindowsCodecs.dll: This is another low level API which is used for imaging support in WPF applications.WindowsCodecs.dll comprises a number of codecs which encode / decode images into vector graphics that would be rendered into WPF screen.

Direct3D: It is the low level API in which the graphics of WPF is rendered.

User32: It is the primary core API which every program uses. It actually manages memory and process separation.

GDI & Device Drivers: GDI and Device Drivers are specific to the operating system which is also used from the application to access low level APIs.

In the above figure, you can see how different framework elements communicate between one another.

Visual Tree and Logical Tree

Every programming style contains some sort of LogicalTree which comprises the Overall Program. TheLogicalTree comprises the elements as they are listed in XAML. Thus they will only include the controls that you have declared in you XAML.

VisualTree on the other hand, comprises the parts that make up the individual controls. You do not generally need to deal with VisualTree directly, but you should know how each control is comprised of, so it would be easier to build custom templates using this.

Dispatcher & Thread Affinity

When WPF application starts, it actually creates two threads automatically. One is Rendering Thread, which is hidden from the programmer, so you cannot use the rendering thread directly from your program; while the other is Dispatcher Thread, which actually holds all the UI elements. So in other words, you might say Dispatcher is actually the UI thread which ties all the elements created within the WPF application. Conversely, WPF requires all the UI elements to be tied with Dispatcher thread, this is called Thread Affinity. Thus you cannot change any element created on Dispatcher thread from any other threads, thus it follows the same Win32 based APIs. Thus it allows you to inter-operate any WPF component into HWNDbased API. For more, read this. [^]

Dispatcher is a class that handles thread affinity. It is actually a prioritized message loop through which all elements are channeled through. Every UIElement is derived from DispatcherObject which defines a property called Dispatcher which points to the UI thread. Thus from any other thread, if you want to invoke or access UI component, you need to Invoke using Dispatcher thread. DispatcherObjectactually has two chief duties, to check and verify if the thread has access to the object.