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On the surface, finalization seems pretty straightforward: you create an object; when the object is collected, the object s Finalize method is called. But once you dig in, finalization is more complicated than this. When an application creates a new object, the new operator allocates the memory from the heap. If the object s type defines a Finalize method, a pointer to the object is placed on the finalization list just before the type s instance constructor is called. The finalization list is an internal data structure controlled by the garbage collector. Each entry in the list points to an object that should have its Finalize method called before the object s memory can be reclaimed. Figure 19 4 shows a heap containing several objects. Some of these objects are reachable from the application s roots, and some are not. When objects C, E, F, I, and J were created, the system detected that these objects types defined Finalize methods and added pointers to these objects to the finalization list.
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Figure 19 4 : The managed heap showing pointers in its finalization list Note Even though System.Object defines a Finalize method, the CLR knows to ignore it; that is, when constructing an instance of a type, if the type s Finalize method is the one inherited from System.Object, the object isn t considered finalizable. One of the derived types must 363
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override Object s Finalize method. When a garbage collection occurs, objects B, E, G, H, I, and J are determined to be garbage. The garbage collector scans the finalization list looking for pointers to these objects. When a pointer is found, the pointer is removed from the finalization list and appended to the freachable queue. The freachable queue (pronounced F reachable ) is another of the garbage collector s internal data structures. Each pointer in the freachable queue identifies an object that is ready to have its Finalize method called. After the collection, the managed heap looks like Figure 19 5.
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Figure 19 5 : The managed heap showing pointers that moved from the finalization list to the freachable queue In this figure, you see that the memory occupied by objects B, G, and H has been reclaimed because these objects didn t have a Finalize method that needed to be called. However, the memory occupied by objects E, I, and J couldn t be reclaimed because their Finalize methods haven t been called yet. A special high priority CLR thread is dedicated to calling Finalize methods. A dedicated thread is used to avoid potential thread synchronization situations that could arise if one of the application s threads was used instead. When the freachable queue is empty (the usual case), this thread sleeps. But when entries appear, this thread wakes, removes each entry from the queue, and calls each object s Finalize method. Because of the way this thread works, you shouldn t execute any code in a Finalize method that makes any assumptions about the thread that s executing the code. For example, avoid accessing thread local storage in the Finalize method. The interaction between the finalization list and the freachable queue is fascinating. First I ll tell you how the freachable queue got its name. Well, the f is obvious and stands for finalization : every entry in the freachable queue should have its Finalize method called. But the "reachable" part of the name means that the objects are reachable. To put it another way, the freachable queue is considered a root just as global and static variables are roots. So if an object is on the freachable queue, the object is reachable and is not garbage. In short, when an object isn t reachable, the garbage collector considers the object garbage. Then when the garbage collector moves an object s entry from the finalization list to the freachable queue, the object is no longer considered garbage and its memory can t be reclaimed. At this point, the garbage collector has finished identifying garbage. Some of the objects identified as garbage have been reclassified as not garbage: in a sense, the object has become resurrected. The garbage 364
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collector compacts the reclaimable memory, and the special CLR thread empties the freachable queue, executing each object s Finalize method. The next time the garbage collector is invoked, it sees that the finalized objects are truly garbage because the application s roots don t point to it and the freachable queue no longer points to it. The memory for the object is simply reclaimed. The important point to get from all of this is that two garbage collections are required to reclaim memory used by objects that require finalization. In reality, more than two collections might be necessary because the objects get promoted to another generation (which I ll explain later). Figure 19 6 shows what the managed heap looks like after the second garbage collection.
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