The notion that you must change the public API is simply not true.
You are right, but when the controls evolve, some APIs have to be replaced with more advanced ones, while other become obsolete because the related features do not need them anymore, etc. In addition, when a control has to receive dramatic changes because the requested features cannot be implemented with the current code base, we usually choose the optimal API for the new control according the developer feedback we received through the years. Of course, in such cases we try to provide temporary backwards compatible APIs to support the older versions for a while.
The WPF controls in the .NET Framework do not change.
This is true, but most of the WPF controls in the .NET Framework are very basic in comparison to RadScheduler, for example, and their feature sets does not change much across the releases. In addition, the .NET Framework is updated once in few years.
The point is for us to not spend time redoing work we have already considered complete!
You are right, but generally if you consider something complete, you should not upgrade its parts that might change over time. We provide bugfixes for older versions, so usually you are not forced to upgrade to a new major version with breaking changes because of existing bugs.
Regarding the RadScheduler theme that has been broken - it is our fault that we introduced a breaking change in a service pack release. This happens rarely, but sometimes we dont have a choice and I apologize for that. This specific problem is caused by a memory leak fix, that we introduced several weeks ago. A leaking property, holding a reference to RadScheduler was removed, hence the themes had to be updated in order to fix a couple of bindings to that property. I can provide more details about the memory leaks we fixed in the recent builds, if you are interested.
The changes in the theme are minor, so if you send us your XAML we will fix it immediately.
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