c# - What are app domains used for?

Question

I understand roughly what an AppDomain is, however I don't fully understand the uses for an AppDomain.

I'm involved in a large server based C# / C++ application and I'm wondering how using AppDomains could improve stability / security / performance.

In particular:

• I understand that a fault or fatal exception in one domain does not affect other app domains running in the same process - Does this also hold true for unmanaged / C++ exceptions, possibly even heap corruption or other memory issues.
• How does inter-AppDomain communication work?
• How is using AppDomains different from simply spawning many processes?

Answer 1

The basic use case for an AppDomain is in an environment that is hosting 3rd party code, so it will be necessary not just to load assemblies dynamically but also unload them.

There is no way to unload an assembly individually. So you have to create a separate AppDomain to house anything that might need to be unloaded. You can then trash and rebuild the whole AppDomain when necessary.

By the way, native code corrupting the heap cannot be protected against by any feature of the CLR. Ultimately the CLR is implemented natively and shares the same address space. So native code in the process can scribble all over the internals of the CLR! The only way to isolate badly behaved (i.e. most) native code is actual process isolation at the OS level. Launch mutiple .exe processes and have them communicate via some IPC mechanism.