Also in Longhorn, Microsoft plans to integrate a replacement for the Windows graphics device interface (GDI), code-named Avalon, that replaces the need to do manual coding with prebuilt, extensible XAML scripts. That means developers wouldn't have to access many APIs directly and instead can modify XAML scripts, sources said.

The Windows GDI currently interacts with device drivers on behalf of Windows applications. The next-generation XAML has new metatags and extensible schemas for user-interface structures and behaviors that are designed to simplify and increase the customization of the "jazzed up and 3-D oriented Longhorn GUI, code-named Aero," sources said.

"It's hard to use the shell now for an application," said another source familiar with the Longhorn plans. "Anything a shell can do, an application can do. So now a Windows application can inherit the behavior of the operating system with zero lines of code."

In addition to the XAML engine, Microsoft plans to integrate an XAML visual designer in the next release of Visual Studio. Net, code-named Whidbey. The company also is slated to ship a Longhorn software developer kit (SDK) that consists of out-of-the-box UI components and behaviors. Microsoft said the new Longhorn SDK will combine the once-separate tools, kits and technical content to enable end-to-end Windows client development.