Hi Juan Pablo,
Currently, we have an issue with the ToolTipText property serialization. This issue will be addressed in one of our next versions.
Regarding your approach, if you manually insert a code line for this property in the designer and make even a small change in the user control, this line will be automatically removed.
Therefore, I would suggest setting the ToolTipText property in the user control's constructor:
| this.radButton1.ButtonElement.ToolTipText = "Open Model";
I hope this helps. If you have additional questions, feel free to contact me.
the Telerik team
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