In SL3 WindowState is located in Telerik.Windows.Controls namespace, but in SL4 we use WindowState enum located in System.Windows namespace. The reason for this is that in SL3 there is no WindowState in the System.Windows namespace and we use our own enum. Unfortunately, the differences between SL3 and SL4 controls are not documented at this stage. We are sorry for any inconvenience caused.
Meanwhile, you could refer to our release notes for the changes in each new version release of Telerik Controls according to the previous ones.
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