Licensing
An April 2007 PC World report suggested that Microsoft intended to release certain parts of Silverlight source code as open source software, but a week later Sam Ramji, director of platform technology strategy at Microsoft, contradicted the rumors by confirming that the company had no plans to open Silverlight. Some controls that ship with Silverlight are available under the Microsoft Public License as a part of a separate project known as the Silverlight Toolkit.
Silverlight's proprietary nature is a concern to competition since it may harm the open nature of the World Wide Web. Advocates of free software are also concerned Silverlight could be another example of Microsoft's embrace, extend and extinguish strategy. Both Microsoft Silverlight and Adobe Flash are proprietary. Flash's file formats are publicly documented standards, as are Silverlight's. However, the communication between a Flash player and a server is done by the proprietary protocol RTMP. Both Flash and Silverlight use patent-encumbered audio and video codecs.
Read more about this topic: Microsoft Silverlight