In an Asynchronous Pluggable Protocol thread - how can this be associated/resolved to a particular COM object implementing ::SetSite()?

multithreading,com,bho,pluggableprotocol

Try the Switch/Continue trick. Your APP would call IInternetProtocolSink::Switch like this, e.g. in Start: PROTOCOLDATA data = {0}; data.grfFlags = PD_FORCE_SWITCH; // important pProtocolSink->Switch(&data); Eventually, the client will turn around and call IInternetProtocol::Continue on you, on the main UI thread. Once on the main thread, things like IServiceProvider::QueryService(IID_IWindowForBindingUI) should work,...