You cannot use Silverlight controls in a WPF application, because they reference different runtime assemblies. As we understand that most developers would need the same controls in both Silverlight and WPF, we decided to postpone the development of some controls for all platforms (ComboBox, Menu, Scheduler etc.) because we needed to focus on bigger and more frequently requested controls, such as the GridView and Charting. On the other hand, some of the standard WPF controls (Menu for example) have so many features and it is very hard to provide something in addition to them.
Our long term goal is to provide the same controls in both suites, because we want to let the developers to use the same source code in both WPF and Silverlight as much as possible. Generally, the decision which control should be developed first for the "other" platform is made depending on the client feedback, so you could change the order by telling us which controls do you need the most.
All the best,
the Telerik team
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