An application domain (AppDomain) is a kind of virtual application. It contains and runs code, starts multiple threads, and links to any needed reference, such as external assemblies or COM libraries.

Application domains can be created to isolate portions of an application and prevent them from directly contacting other portions; to configure different kinds of security authorizations, such as with Code Access Security (CAS) techniques to limit I/O access, network access, and so on; or to simply increment the whole security level of the application by isolating different application contexts from others.

AppDomain usage for reference and/or CAS isolation

The application domains can be unloaded if needed, allowing us to work in a reliable way with multiple external plugins or extensions, like the ones from IoC designs, or simply because we need to load multiple versions of the same assembly all together.

External libs loaded at runtime in temporary AppDomains

Application...