Summary

Application domains are often described as subprocesses because they provide an isolation boundary around a grouping of assemblies in a process. If the code running in the application domain is verifiably type safe, the isolation provided by the domain is roughly equivalent to that provided by a process in Win32. Application domains also provide the ability to unload code from a running process dynamically. As the author of an extensible application, one of the key decisions you'll make is how to partition your process into multiple application domains. The key is to use application domains to achieve the isolation you want while minimizing the amount of communication between domains because of the cost associated with remote calls.

When writing an extensible application, you'll likely want to take advantage of a new concept in .NET Framework version 2.0 called an application domain manager. Providing an application domain manager is an easy way to implement the host runtime design pattern because the CLR automatically creates an instance of your domain manager in every application domain. Also, if you're writing a CLR host, an application domain manager is the mechanism you use to call into managed code from the unmanaged portion of your host.