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Windows Presentation Foundation in .NET 4 : Introducing WPF - The Evolution of Windows Graphics & A Higher-Level API

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8/15/2011 6:21:44 PM
The Windows Presentation Foundation (WPF) is a graphical display system for Windows. WPF is designed for .NET, influenced by modern display technologies such as HTML and Flash, and hardware-accelerated. It's also the most radical change to hit Windows user interfaces since Windows 95.

1. The Evolution of Windows Graphics

It's hard to appreciate how dramatic WPF is without realizing that Windows developers have been using essentially the same display technology for more than 15 years. A standard Windows application relies on two well-worn parts of the Windows operating system to create its user interface:

  • User32. This provides the familiar Windows look and feel for elements such as windows, buttons, text boxes, and so on.

  • GDI/GDI+. This provides drawing support for rendering shapes, text, and images at the cost of additional complexity (and often lackluster performance).

Over the years, both technologies have been refined, and the APIs that developers use to interact with them have changed dramatically. But whether you're crafting an application with .NET and Windows Forms or lingering in the past with Visual Basic 6 or MFC-based C++ code, behind the scenes the same parts of the Windows operating system are at work. Newer frameworks simply deliver better wrappers for interacting with User32 and GDI/GDI+. They can provide improvements in efficiency, reduce complexity, and add prebaked features so you don't have to code them yourself; but they can't remove the fundamental limitations of a system component that was designed more than a decade ago.


The basic division of labor between User32 and GDI/GDI+ was introduced more than 15 years ago and was well established in Windows 3.0. Of course, User32 was simply User at that point, because software hadn't yet entered the 32-bit world.

1.1. DirectX: The New Graphics Engine

Microsoft created one way around the limitations of the User32 and GDI/GDI+ libraries: DirectX. DirectX began as a cobbled-together, error-prone toolkit for creating games on the Windows platform. Its design mandate was speed, and so Microsoft worked closely with video card vendors to give DirectX the hardware acceleration needed for complex textures, special effects such as partial transparency, and three-dimensional graphics.

Over the years since it was first introduced (shortly after Windows 95), DirectX has matured. It's now an integral part of Windows, with support for all modern video cards. However, the programming API for DirectX still reflects its roots as a game developer's toolkit. Because of its raw complexity, DirectX is almost never used in traditional types of Windows applications (such as business software).

WPF changes all this. In WPF, the underlying graphics technology isn't GDI/GDI+. Instead, it's DirectX. Remarkably, WPF applications use DirectX no matter what type of user interface you create. That means that whether you're designing complex three-dimensional graphics (DirectX's forte) or just drawing buttons and plain text, all the drawing work travels through the DirectX pipeline. As a result, even the most mundane business applications can use rich effects such as transparency and anti-aliasing. You also benefit from hardware acceleration, which simply means DirectX hands off as much work as possible to the graphics processing unit (GPU), which is the dedicated processor on the video card.


DirectX is more efficient because it understands higher-level ingredients such as textures and gradients that can be rendered directly by the video card. GDI/GDI+ doesn't, so it needs to convert them to pixel-by-pixel instructions, which are rendered much more slowly by modern video cards.

One component that's still in the picture (to a limited extent) is User32. That's because WPF still relies on User32 for certain services, such as handling and routing input and sorting out which application owns which portion of screen real estate. However, all the drawing is funneled through DirectX.


This is the most significant change in WPF. WPF is not a wrapper for GDI/GDI+. Instead, it's a replacement—a separate layer that works through DirectX.

1.2. Hardware Acceleration and WPF

You're probably aware that video cards differ in their support for specialized rendering features and optimizations. Fortunately, this isn't a problem, because WPF has the ability to perform everything it does using software calculations rather than relying on built-in support from the video card.


There's one exception to WPF's software support. Because of poor driver support, WPF performs anti-aliasing for 3-D drawings only if you're running your application on Windows Vista or Windows 7 (and you have a native WDDM driver for your video card). That means that if you draw three-dimensional shapes on a Windows XP computer, you'll end up with slightly jagged edges rather than nicely smoothed lines. However, anti-aliasing is always provided for 2-D drawings, regardless of the operating system and driver support.

Having a high-powered video card is not an absolute guarantee that you'll get fast, hardware-accelerated performance in WPF. Software also plays a significant role. For example, WPF can't provide hardware acceleration to video cards that are using out-of-date drivers. (If you're using an older video card, these out-of-date drivers are quite possibly the only ones that were provided in the retail package.) WPF also provides better performance under the Windows Vista and Windows 7 operating systems, where it can take advantage of the Windows Display Driver Model (WDDM). WDDM offers several important enhancements beyond the Windows XP Display Driver Model (XPDM). Most importantly, WDDM allows several GPU operations to be scheduled at once, and it allows video card memory to be paged to normal system memory if you exceed what's available on the video card.

As a general rule of thumb, WPF offers some sort of hardware acceleration to all WDDM drivers and to XPDM drivers that were created after November 2004, which is when Microsoft released new driver development guidelines. Of course, the level of support differs. When the WPF infrastructure first starts up, it evaluates your video card and assigns it a rating from 0 to 2, as described in the sidebar "WPF Tiers."

Part of the promise of WPF is that you don't need to worry about the details and idiosyncrasies of specific hardware. WPF is intelligent enough to use hardware optimizations where possible, but it has a software fallback for everything. So if you run a WPF application on a computer with a legacy video card, the interface will still appear the way you designed it. Of course, the software alternative may be much slower, so you'll find that computers with older video cards won't run rich WPF applications very well, especially ones that incorporate complex animations or other intense graphical effects. In practice, you might choose to scale down complex effects in the user interface, depending on the level of hardware acceleration that's available in the client (as indicated by theRenderCapability.Tier property).


The goal of WPF is to off-load as much of the work as possible on the video card so that complex graphics routines are render-bound (limited by the GPU) rather than processor-bound (limited by your computer's CPU). That way, you keep the CPU free for other work, you make the best use of your video card, and you are able to take advantage of performance increases in newer video cards as they become available.

WPF Tiers

Video cards differ significantly. When WPF assesses a video card, it considers a number of factors, including the amount of RAM on the video card, support for pixel shaders (built-in routines that calculate per-pixel effects such as transparency), and support for vertex shaders (built-in routines that calculate values at the vertexes of a triangle, such as the shading of a 3-D object). Based on these details, it assigns a rendering tier value.

WPF recognizes three rendering tiers:

  • Rendering Tier 0. The video card will not provide any hardware acceleration. This corresponds to a DirectX version level of less than 7.0.

  • Rendering Tier 1. The video card can provide partial hardware acceleration. This corresponds to a DirectX version level greater than 7.0 but less than 9.0.

  • Rendering Tier 2. All features that can be hardware accelerated will be. This corresponds to a DirectX version level greater than or equal to 9.0.

In some situations, you might want to examine the current rendering tier programmatically so you can selectively disable graphics-intensive features on lesser-powered cards. To do so, you need to use the static Tier property of the System.Windows.Media.RenderCapability class. But there's one trick. To extract the tier value from the Tier property, you need to shift it 16 bits, as shown here:

int renderingTier = (RenderCapability.Tier >> 16);

if (renderingTier == 0)
{ ... }
else if (renderingTier == 1)
{ ... }

This design allows extensibility. In future versions of WPF, the other bits in the Tier property might be used to store information about support for other features, thereby creating subtiers.

For more information about what WPF features are hardware-accelerated for tier 1 and tier 2 and for a list of common tier 1 and tier 2 video cards, refer to http://msdn.microsoft.com/en-us/library/ms742196(VS.100).aspx.

2. WPF: A Higher-Level API

If the only thing WPF offered was hardware acceleration through DirectX, it would be a compelling improvement but not a revolutionary one. But WPF actually includes a basket of high-level services designed for application programmers.

The following are some of the most dramatic changes that WPF ushers into the Windows programming world:

  • A web-like layout model. Rather than fix controls in place with specific coordinates, WPF emphasizes flexible flow layout that arranges controls based on their content. The result is a user interface that can adapt to show highly dynamic content or different languages.

  • A rich drawing model. Rather than painting pixels, in WPF you deal with primitives—basic shapes, blocks of text, and other graphical ingredients. You also have new features, such as true transparent controls, the ability to stack multiple layers with different opacities, and native 3-D support.

  • A rich text model. After years of substandard text handling, WPF finally gives Windows applications the ability to display rich, styled text anywhere in a user interface. You can even combine text with lists, floating figures, and other user interface elements. And if you need to display large amounts of text, you can use advanced document display features such as wrapping, columns, and justification to improve readability.

  • Animation as a first-class programming concept. In WPF, there's no need to use a timer to force a form to repaint itself. Instead, animation is an intrinsic part of the framework. You define animations with declarative tags, and WPF puts them into action automatically.

  • Support for audio and video media. Previous user interface toolkits, such as Windows Forms, were surprisingly limited when dealing with multimedia. But WPF includes support for playing any audio or video file supported by Windows Media Player, and it allows you to play more than one media file at once. Even more impressively, it gives you the tools to integrate video content into the rest of your user interface, allowing you to pull off exotic tricks such as placing a video window on a spinning 3-D cube.

  • Styles and templates. Styles allow you to standardize formatting and reuse it throughout your application. Templates allow you to change the way any element is rendered, even a core control such as the button. It's never been easier to build modern skinned interfaces.

  • Commands. Most users realize that it doesn't matter whether they trigger the Open command through a menu or through a toolbar; the end result is the same. Now that abstraction is available to your code, you can define an application command in one place and link it to multiple controls.

  • Declarative user interface. Although you can construct a WPF window with code, Visual Studio takes a different approach. It serializes each window's content to a set of XML tags in a XAML document. The advantage is that your user interface is completely separated from your code, and graphic designers can use professional tools to edit your XAML files and refine your application's front end.

  • Page-based applications. Using WPF, you can build a browser-like application that lets you move through a collection of pages, complete with forward and back navigation buttons. WPF handles the messy details such as the page history. You can even deploy your project as a browser-based application that runs right inside Internet Explorer.

2.1. Windows Forms Lives On

WPF is the platform for the future of Windows user interface development. However, it won't displace Windows Forms overnight. Windows Forms is in many ways the culmination of the previous generation of display technology, which was built on GDI/GDI+ and User32.

So, which platform should you choose when you begin designing a new Windows application? If you're starting from the ground up, WPF is an ideal choice, and it offers the best prospects for future enhancements and longevity. Similarly, if you need one of the features that WPF provides and Windows Forms does not—such as 3-D drawing or page-based applications—it makes sense to make the shift. On the other hand, if you have a considerable investment in a Windows Forms–based business application, there's no need to recode your application for WPF. The Windows Forms platform will continue to be supported for years to come.

Perhaps the best part of the story is that Microsoft has invested considerable effort in building an interoperability layer between WPF and Windows Forms (which plays a similar role to the interoperability layer that allows .NET applications to continue to use legacy COM components).

2.2. DirectX Also Lives On

There's one area where WPF isn't a good fit—when creating applications with demanding real-time graphics, such as complex physics-based simulators or cutting-edge action games. If you want the best possible video performance for these types of applications, you'll need to program at a much lower level and use raw DirectX. You can download the managed .NET libraries for DirectX programming at http://msdn.microsoft.com/directx.

2.3. Silverlight

Like the .NET Framework, WPF is a Windows-centric technology. That means that WPF applications can be used only on computers running the Windows operating system. Browser-based WPF applications are similarly limited—they can run only on Windows computers, although they support both the Internet Explorer and Firefox browsers.

These restrictions won't change—after all, part of Microsoft's goal with WPF is to take advantage of the rich capabilities of Windows computers and its investment in technologies such as DirectX. However, Silverlight is designed to take a subset of the WPF platform, host it in any modern browser using a plug-in (including Firefox, Google Chrome, and Safari), and open it up to other operating systems (such as Linux and Mac OS). This is an ambitious project that's attracted considerable developer interest.

In many ways, Silverlight is based on WPF, and it incorporates many of WPF's conventions . However, Silverlight also leaves out certain feature areas, such as true three-dimensional drawing or rich document display. New features may appear in future Silverlight releases, but the more complex ones might never make the leap.

The ultimate goal of Silverlight is to provide a powerful developer-oriented competitor for Adobe Flash. However, Flash has a key advantage—it's used throughout the Web, and the Flash plug-in is installed just about everywhere. To entice developers to switch to a new, less-established technology, Microsoft will need to make sure Silverlight has next-generation features, rock-solid compatibility, and unrivaled design support.

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