As described in our documentation, the Telerik Presentation Framework is the framework on top of which our RadControls are built. The Framework consists of primitives (painting fill, border, text), layouts (grid layout, stack layout, etc.) and API that supports our styling. The API that gives you access to the described Framework parts is contained and provided by the Telerik.WinControls.dll assembly that you can find in the bin folder of the RadControls for WinForms distribution. Despite the great powers of the Telerik Presentation Framework, it may take you some time until you craft a component that perfectly suits your needs. Therefore, I would recommend first checking the implementation of the expanders built for the purposes of our new Demo application. It can be found in the QuickStart project that comes with our Source Code. Please note, however, that these expanders are not an official part of the RadControls for WinForms Toolbox, thus they come as they are and support for them is not available.

The Source Code itself is available to customers with the appropriate licenses. In case you have the appropriate license, you can find the Source Code at your Telerik account >> Products & Subscriptions >> DevCraft >> Download Installer and Other Resources >> RadControls for WinForms >> Browse all product files >> Source Code.

I hope this helps.