Windows Script Hosts
When implementing Microsoft Windows Script host, you can safely assume that a scripting engine only calls the IActiveScriptSite interface in the context of the base thread as long as the host does the following:
Chooses a base thread (generally the thread that contains the message loop).
Instantiates the scripting engine in the base thread.
Calls the scripting engine's dispatch object only from the base thread.
Ensures that the message loop runs in the base thread if a window handle is provided.
Ensures that objects in the host's object model only source events in the base thread.
These rules are automatically followed by all single-threaded hosts. The restricted model described above is intentionally loose enough to allow a host to abort a stuck script by calling IActiveScript::InterruptScriptThread from another thread (initiated by a CTRL+BREAK handler or the like), or to duplicate a script in a new thread using IActiveScript::Clone.
None of these restrictions apply to a host that chooses to implement a free-threaded IActiveScriptSite interface and a free-threaded object model. Such a host can use the IActiveScript interface from any thread, without restriction.