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.

Answer 2

I highly recommend CLR Via C# by Jeffrey Richter. In particular chapter 21 goes into good detail regarding the purpose and uses of AppDomains.

In answer to your points/question:

• AppDomains will not protect your application from rogue unmanaged code. If this is an issue you will most likely need to use full process isolation provided by the OS.

• Communication between AppDomains is performed using .NET remoting to enforce isolation. This can be via marshal by reference or marshal by value semantics, with a trade off between performance and flexibility.

• AppDomains are a lightweight way of achieving process like isolation within managed code. AppDomains are considered lightweight because you can create multiple AppDomains within a single process and so they avoid the resource and performance overhead multiple OS processes. Also, a single thread can execute code in one AppDomain and then in another AppDomain as Windows knows nothing about AppDomains (see this by using using System.AppDomain.CurrentDomain)

Answer 3

Actually, it is not true that a critical fail in one AppDomain can't impact others. In the case of bad things, the best bet it to tear down the process. There are a few examples, but to be honest I haven't memorised them - I simply took a mental note "bad things = tear down process (check)"

Benefits of AppDomain:

• you can unload an AppDomain; I use this for a system that compiles itself (meta-programming) based on data from the database - it can spin up an appdomain to host the new dll for a while, and then swap it safely when new data is available (and built)
• comms between AppDomains are relatively cheap. IMO this is the only time I am happy to use remoting (although you still need to be really careful about the objects on the boundary to avoid bleeding references between them, causing "fusion" to load extra dlls into the primary AppDomain, causing a leak) - it is really easy too - just CreateInstanceAndUnwrap (or is it CreateInstanceFromAndUnwrap?).
• vs spawing an extra process - you could go either way; but you don't need another exe for AppDomain work, and it is much easier to set up any comms that you need