Perhaps we should add module destructors as well?

[cheong00]

Since we will likely have module initializers on C#9, and the rule don't forbid the use of unmanaged resources in there, perhaps it make sense to also add "module destructors" for teardown?

[huoyaoyuan]

Module initializer wasn't added, but exposed from C#. It requires IL having the feature first.

[sharwell]

AppDomain.Unload is probably the closest we have to a module destructor. It has proven extremely unreliable in practice. If a module needs to run code during application shutdown, it needs to be a normal part of code flow and cannot be a user-defined finalizer. Otherwise in practice you'll end up with 25% success, 25% silently fails to run, 25% deadlock that leaves the process running in the background, and 25% crash.

[Joe4evr]

I believe this is relevant here: https://devblogs.microsoft.com/oldnewthing/20120105-00/?p=8683

Don’t worry about freeing memory; it will all go away when the process address space is destroyed. Don’t worry about closing handles; handles are closed automatically when the process handle table is destroyed.

[qrli]

For unmanaged DLL, the support is even weaker, as unmanaged world can play a lot of tricks and they do. E.g. some other software (e.g. anti-virus) may hook the DLL API and change the timing and order things happened.

As @huoyaoyuan mentioned, it is a limitation of .NET runtime rather than C#. But I can kind of imagine why such support was not added, from the history of various issues happened with DLL_PROCESS_DETACH. It is very hard to do right in practice (most things no longer work there), similar to finalizer. Even finalizers have been discouraged for long, it is better not to let applications to play with DLL unload hook.

But you are also right that there are cases it is indeed necessary. Then it boils down to RoI for such feature. Without such a feature, we can still make an explicit normal function call to clean up, which is more controllable on timing and threading. It may not be bad at all.

[Joe4evr]

If I understand the way SafeHandles work correctly, wouldn't something kinda like this work to accomplish this from the managed side?

[cheong00]

Okay, it should work. Just not something that newer users of the language possibly able to figure out immediately.

[Joe4evr]

Well, newer users to the language shouldn't be directly using advanced interop features like SafeHandles and ModuleInitializers, anyway. 🙃

[jnm2]

Okay, it should work. Just not something that newer users of the language possibly able to figure out immediately.

@BillWagner Is this helpful to track for potential docs?