I already answered your other thread - for your convenience I am placing my answer below:
The Controls collection is inherited from the Control class. The RadSplitter
control is a complex control and its child controls are also third part user controls and according to some good practices it's comfortable to use another alternative collection for them. Considering the RadSlidingZone
this collection is its Items collection. This being said, you should add the RadSlidingPanes
in the Items collection as shown below:
We apologize for the inconvenience and we logged this in our TODO list - we will write additional code in order to make this controls available in the Controls collection too.
Your points were updated.
the Telerik team