What is an AppDomain? What are the benefits of AppDomains or why Microsoft brought the concept of AppDomains, what was the problem without AppDomains?
Please elaborate.
3 Answers
An AppDomain provides a layer of isolation within a process. Everything you usually think of as "per program" (static variables etc) is actually per-AppDomain. This is useful for:
- plugins (you can unload an
AppDomain, but not an assembly within anAppDomain) - security (you can run a set of code with specific trust levels)
- isolation (you can run different versions of assemblies etc)
The pain is you need to use remoting etc.
See MSDN for lots more info. To be honest, it isn't something you need to mess with very often.
AppDomains can be viewed as lightweight processes. They share many of the same characteristics of a process, e.g. they have their own copies of statics, assemblies and so forth, but they are contained within a single process. From the operating system's point of view a process is just a process no matter how many AppDomains it may contain.
Unlike a process however, an AppDomain does not have any threads unless you explicitly create them. A thread can run code in any AppDomain.
AppDomains are part of the same process and thus actually share the same managed heap. This is usually not an issue since the AppDomain programming model prevents implicit access between AppDomains. However, some references are actually shared between AppDomains such as type objects and interned strings.
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