Thank you for explaining your point.
When you create a designer for the datepicker which is using DatePicker namespace where the control is created you should really use the same namespace if you want to create a designer for that same control.
However I understand that tutorial can be followed for just a reference to creating a control designer for some other custom created control and not mentioning this may really cause a slowdown. We will review it with our documentation team and correct it.
Thank you for the valuable observation.
the Telerik team
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