I posted a blog about AddIn Adapters here.

There is a question. Can we have AddInAdapters inside HostAdapter?

This is a tricky one. It depends on what the definition of AddInAdapter is.

The answer is Yes. If AddInAdapter refers to any type inherited from ContractBase.

The answer is No. If AddInAdapter refers to any type with AddInAdapterAttribute so that the Host can discover and activate.