AppDomain, Thread and process

Application domain (other name appDomain) is nothing but one logical region where .NET runtime runs and execute code.  It provides security and isolation for executing managed code.

We can use application domain to isolate any task that might bring down process. From application domain we can unload any task without affecting the process. We can also use application domains to isolate tasks that should not share data. In this way application domain gives security to different process.

Multiple application domains can run a single process and there might be more than one thread in single application domain.

Multiple application domains can exist in one Win32 process. As we have discussed earlier that the primary goal of application domain is to separate or isolate process and it is very similar with different processes in one operating system. The .NET runtime force application domain isolation by keeping control over memory.