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.