Microsoft is making a big platform bet on .NET as a default application runtime.

At MIX 2010 in Las Vegas yesterday, the company revealed that Windows Phone 7 will require programmers to use Silverlight and XNA for development.

Windows Phone 7’s Silverlight implementation will include aspects of Silverlight 4, said Larry Lieberman, senior product manager for Windows Mobile developer experience. Microsoft released a Silverlight 4 release candidate at MIX.

Lieberman described the Windows Phone’s use of managed code as a “dramatic change” in how Microsoft created software, as well as a “clean slate” for developers. There will be no backward compatibility with Windows CE applications in Windows Phone 7.

“We asked ourselves, ‘What do we need to do to compete in this vastly changed [mobile] landscape?’ ” Lieberman said. “What we did was determine to focus on end-user experiences.” Silverlight and XNA were the assets Microsoft has to compete with, he added.

Using Silverlight will foster the development of thousands of compelling applications for Windows Phone 7 within a short period of time, Lieberman said.

Over 500,000 Silverlight developers “became” Windows Phone 7 developers overnight, claimed Brian Goldfarb, director of developer and user experience platforms at Microsoft, on a call with the press. Goldfarb predicted that Silverlight would have 80% market penetration on Internet-connected devices by this summer.

The statistic on the number of Silverlight developers worldwide was announced last year at MIX 09, and is based upon internal developer tracking data from Microsoft.

“The Silverlight skills are the same. The developers are the same. I am not sure what the number of [Silverlight] developers are, but potentially, in the long run, the entire .NET developer base has relevant skills that they can leverage,” said Al Hilwa, program director of application development software at IDC.

Lieberman noted that developers would still have to tailor their Silverlight applications to the Windows Phone 7 platform. “We’re not positioning this as a ‘write once/run everywhere’ solution,” he said. “We think this vastly accelerates the ability to create compelling applications for the phone.”

Microsoft is providing free tooling for developers, including Microsoft Visual Studio 2010 Express for Windows Phone, Windows Phone 7 Series Emulator for application testing, and XNA Game Studio 4.0. A technology preview of Expression Blend for Windows Phone Community is also available.

A beta of Expression Blend 4, Microsoft’s design and development workflow tool, was also announced at MIX. It allows interfaces to be animated without coding, and adds support for Silverlight 4, .NET Framework 4 and Visual Studio 2010.

Expression Blend will open up mobile development to graphic designers, Lieberman said. Microsoft will provide developers with guidelines for integrating their applications “as tightly as possible” with Windows 7’s user interface, he added. Visual Studio 2010 Express will include themes and templates for the OS.

Windows Phone 7’s “Metro” interface is a new UI concept from Microsoft. It is designed to help users “find their way” across their phones and information on the Web, according to a Metro book set that Microsoft distributed at MIX.

Microsoft will provide a push notification server to developers for their applications, which enables end users to receive updates from their applications even when those applications are not running. Restricting how many applications that can run preserves battery life and simplifies the user experience, according to a blog post by Microsoft developer liaison Charlie Kindel. Initially, only Microsoft’s own applications will be given full multitasking capabilities.

With its focus on making .NET the application runtime for Windows Phone, Microsoft allocated fewer resources toward the OS’ browser, Lieberman acknowledged. The browser is based on Internet Explorer 7.0 and will not support HTML5, which has a framework for Web applications without requiring runtimes.

Android’s Chrome browser and iPhone’s Safari implement HTML5. Lieberman claimed that the browsing experience would be “comparable” with those platforms. However, IE7 fails the Web Standards Project’s tests, including Acid2 and Acid3. The tests are designed to expose Web page rendering flaws in browsers, indicating standards compliance.

Lieberman would not say whether Microsoft has plans to port Silverlight to Android, saying, “The team is always investigating new platforms.” He added that Visual Studio’s extensibility permits third parties to create plug-ins that target platforms that Microsoft does not support.

Novell, a Microsoft partner, is creating a Visual Studio add-on to target Android, along with another tool for translating .NET applications to native code to execute on the iPhone.

“Mobile is a potential strong point for HTML5, and if Windows Phone 7 didn’t support it, then it would certainly tend to limit the Web App approach and push a dev toward using [Silverlight] instead,” said Jeffrey Hammond, principal analyst for Forrester Research.