By default RadRichTextBox uses MEF to load its extensible components like format providers (needed for rich-text copy/paste) and UI widgets (dialogs etc.).
MEF searches types as defined by the ComposablePartCatalog
set to RadCompositionInitializer.Catalog, which by default is Telerik.Windows.Documents.UI.Extensibility.DependencyCatalog. Unfortunately this catalog don't support custom probing paths, which lead to the behavior you are observing.
As a workaround, you can try to implement your own custom catalog which ensures the loading of the wanted types. The simplest possible variant is to use TypeCatalog
, as described in this help article
. Including only the types that you will actually need will ensure performance even better than the default one. The downside is that new types included in future versions of the RadRichTextBox (i.e. after upgrading) won't be automatically loaded. The types that you should include to enable rich text copy/paste are RtfFormatProvider and HtmlFormatProvider.
Don't hesitate to contact us if you have other questions.
Check out the new Telerik Platform - the only modular platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native apps. Register for the free online keynote and webinar to learn more about the Platform on Wednesday, February 12, 2014 at 11:00 a.m. ET (8:00 a.m. PT).