Hi Pan Hal,
Setting control's style in code-behind shouldn't cause any problems. Can you please elaborate on the scenario you need to implement? if you can send us a sample of your code, we will be able to further investigate the issue you are facing.
I attached an example of how you can set the style for RadButton and RadTreeView in code-behind. Can you please take a look at it and modify it to illustrate your scenario?
Let me know how it goes.
the Telerik team
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