Question

If I run this code, will each AppDomain execute in a different thread?

 ThreadPool.QueueUserWorkItem(delegate
 {
     /// Create AppDomain and run code
 });

Answer 1

AppDomains do not get their own thread per default. You may execute code in another AppDomain using existing threads or call a method in the AppDomain, that creates new thread(s). In fact, unless you specifically creates additional threads calling code in another domain will execute on the process’ main thread.

From the AppDomain documentation

Multiple application domains can run
in a single process; however, there is
not a one-to-one correlation between
application domains and threads.
Several threads can belong to a single
application domain, and while a given
thread is not confined to a single
application domain, at any given time,
a thread executes in a single
application domain.

In your example, you create threads (or more specifically the thread pool does so) and thus the code will run on these threads. However, I am not sure I would recommend creating AppDomains on thread pool threads like that.

Unloading an AppDomain will abort any threads in the AppDomain. I honestly don’t know how the thread pool will react to this. More info about unloading here.