Good example of use of AppDomain

Question

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?

Answer 1

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/

Answer 2

Another benefit of AppDomains (as you mentioned in your question) is code that you load into it can run with different security permissions. For example, I wrote an app that dynamically loaded DLLs. I was an instructor and these were student DLLs I was loading. I didn't want some disgruntled student to wipe out my hard drive or corrupt my registry, so I loaded the code from their DLLs into a separate AppDomain that didn't have file IO permissions or registry editing permissions or even permissions to display new windows (it actually only had execute permissions).