Usage of AppDomain in C#

Question

What is the most important use of AppDomains in C#?

Answer 1

The single most important use is that your code has to have one - i.e. everything you write in C# executes in an AppDomain. That is quite important ;-p

If you mean additional app-domains:

When using plugins and other untrusted code, it allows you both isolation, and the ability to unload them (you can't unload assemblies - only entire app-domains).

I'm using it currently to load dynamically generated dlls, so that I can unload them.

They also allow you to set different configuration files, trust levels, etc - but have associated costs of complexity and remoting.

MSDN has a section on app-domains, here.

Answer 2

I can't tell you what the most important use is, since that depends on the situation.

AppDomains are useful for sandboxing parts of your application. You can load extensions in an AppDomain and unload them again - something you cannot otherwise do. You can assign specific rights to AppDomains. Per default objects in different AppDomains cannot access each other.

AppDomains can be viewed as lightweight processes as they give you many of the same features. However, unlike a Process new AppDomains do not have their own thread per default. You have to manage AppDomains and threads yourself.

Also, AppDomains all share the same managed heap. This is usually not a problem, but it may have surprising effects as some instances like strings are shared among AppDomains. For regular use this is not an issue, but if you use strings for locking, threads in different AppDomain can affect each other.