Good example of use of AppDomain

I keep getting asked about AppDomains in interviews, and I know the basics:

they are an isolation level within an application (making them different from applications)
they can have threads (making them different from threads)
exceptions in one appdomain do not affect another
appdomains cannot access each other's memory
each appdomain can have different security

I still don't get what makes them necessary. I'm looking for a reasonable concrete circumstance when you would use one.

Answers:

Untrusted code

Core application protected
Untrusted/3rd party plugins are barred from corrupting shared memory and non-authorized access to registry or hard drive by isolation in separate appdomain with security restrictions, protecting the application or server. e.g. ASP.NET and SQL Server hosting component code

Trusted code

Stability
Application segmented into safe, independent features/functionality
Architectural flexibility
Freedom to run multiple applications within a single CLR instance or each program in its own.

Anything else?

Solution

Probably the most common one is to load assemblies that contain plug-in code from untrusted parties. The code runs in its own AppDomain, isolating the application.

Also, it's not possible to unload a particular assembly, but you can unload AppDomains.

For the full rundown, Chris Brumme had a massive blog entry on this:

~~http://blogs.msdn.com/cbrumme/archive/2003/06/01/51466.aspx~~

https://devblogs.microsoft.com/cbrumme/appdomains-application-domains/