Anapplication domain is a mechanism (similar to aprocess in anoperating system) used within theCommon Language Infrastructure (CLI) to isolate executedsoftware applications from one another so that they do not affect each other. Each application domain has its ownvirtual address space whichscopes the resources for the application domain using thataddress space.
Creating multiple application domains in the same process is not possible in .NET Core and .NET 5+.[1]
A CLI application domain is contained within an operating system process. A process may contain many application domains. Application domains have isolation properties similar to that of operating system processes:
In this sense, a CLI is like a mini-operating system. It runs a single process that contains a number of sub-processes, or application domains.
The advantage of application domains is that running multiple application domains may require fewer resources, such as memory, than running multiple operating system processes. Communication between domains still requires marshalling, so the overheads can be closer to using multiple processes than to communicating within a single domain.
Direct communication cannot be achieved across application domains. However, application domains can still talk to each other by passingobjects viamarshalling by value (unbound objects), marshalling by reference through a proxy (application-domain-bound objects). There is a third type of object called a context-bound object which can be marshalled by reference across domains and also within the context of its own application domain. Because of the verifiabletype-safety ofmanaged code, a CLI can provide fault isolation between domains at a much lower cost than an operating system process can. The static type verification used for isolation does not require the same process switches or hardware ring transitions that an operating system process requires.
Application domains are a purelymanaged code concept. Any includednative/unmanaged code (e.g., C++) is largely unaware of them. Static variables seem to be shared across domains, callbacks can be problematic, and any memory corruption bugs in one domain is likely to corrupt other domains.
