Nbsp;   AppDomain Unloading

One of the great features of AppDomains is that you can unload them. Unloading an AppDomain causes the CLR to unload all of the assemblies in the AppDomain, and the CLR frees the AppDomain’s loader heap as well. To unload an AppDomain, you call AppDomain’s Unload static method (as the Ch22-1-AppDomains application does). This call causes the CLR to perform a lot of actions to grace- fully unload the specified AppDomain:

1.The CLR suspends all threads in the process that have ever executed managed code.

2.The CLR examines all of the threads’ stacks to see which threads are currently executing code in the AppDomain being unloaded, or which threads might return at some point to code in the AppDomain that is being unloaded. The CLR forces any threads that have the unloading AppDomain on their stack to throw a ThreadAbortException (resuming the thread’s execu- tion). This causes the threads to unwind, executing any finally blocks on their way out so that cleanup code executes. If no code catches the ThreadAbortException, it will eventually become an unhandled exception that the CLR swallows; the thread dies, but the process is allowed to continue running. This is unusual, because for all other unhandled exceptions, the CLR kills the process.

3.After all threads discovered in step 2 have left the AppDomain, the CLR then walks the heap and sets a flag in each proxy object that referred to an object created by the unloaded App- Domain. These proxy objects now know that the real object they referred to is gone. If any code now calls a method on an invalid proxy object, the method will throw an AppDomain­ UnloadedException.

4.The CLR forces a garbage collection to occur, reclaiming the memory used by any objects that were created by the now unloaded AppDomain. The Finalize methods for these objects are called, giving the objects a chance to clean themselves up properly.

5.The CLR resumes all of the remaining threads. The thread that called AppDomain.Unload will now continue running; calls to AppDomain.Unload occur synchronously.

My Ch22-1-AppDomains application uses just one thread to do all of the work. Whenever my code calls AppDomain.Unload, there are no threads in the unloading AppDomain, and therefore, the CLR doesn’t have to throw any ThreadAbortException exceptions. I’ll talk more about ThreadAbort­ Exception later in this chapter.

By the way, when a thread calls AppDomain.Unload, the CLR waits 10 seconds for the threads in the unloading AppDomain to leave it. If after 10 seconds, the thread that called AppDomain.Unload doesn’t return, it will throw a CannotUnloadAppDomainException, and the AppDomain may or may not be unloaded in the future.

Date: 2016-03-03; view: 672