A thread is a virtual processor that can run some c3ode from any AppDomain. Although at any single time a thread can run code from a single domain, it can cross domain boundaries when needed. Indeed, in the preceding example, there is only a single thread that did the entire job for all three AppDomains.

Without diving into the internals of Windows threading, we should know that a CLR thread is actually a Windows thread. This one has a high creation cost (even worse for 64-bit systems) as in any other virtualization technique, although in Windows, creating a process is even worse in terms of resource usage. This is why Microsoft has supported multi-threading programming in its operating systems since the age of Windows NT.

What is important to know is that a thread never has 100 percent of CPU's time because its CPU time is reassigned to any new pending-for-work threads every 30 milliseconds (a time-slice) by acting what we call in Windows a context switch.

This ensures that at the...