We have seen this topic as Gabe Sumner has indicated for this and we will try to come up with some document that explains how to extend OA and Sitefinity or Gabe will prepare a blog post/webinar
We want to use telerik classes like the UserManager and not have to duplicate any of their functionality. Ideally we would like all our data to be accessed through our business layer so that all calls to the data look similar and so that we can do business rule validation.
You can take a look at the API reference
to see all methods and properties so you will not duplicate any logic. Your requirement will work only in a custom solution and basically some inheritance from base classes will require more coding. There are internal dependencies and casts that expect certain objects.
Generally you can extend each class and override each virtual method or property and implement your custom logic after the base or skip the base.
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