Terminology[ edit ] The terminology of "finalizer" and "finalization" versus "destructor" and "destruction" varies between authors and is sometimes unclear. For languages that implement garbage collection via reference counting , terminology varies, with some languages such as Objective-C and Perl using "destructor", and other languages such as Python using "finalizer" per spec, Python is garbage collected, but the reference CPython implementation since its version 2. This reflects the fact that reference counting results in semi-deterministic object lifetime: for objects that are not part of a cycle, objects are destroyed deterministically when the reference count drops to zero, but objects that are part of a cycle are destroyed non-deterministically, as part of a separate form of garbage collection. In certain narrow technical usage, "constructor" and "destructor" are language-level terms, meaning "methods defined in a class", while "initializer" and "finalizer" are implementation-level terms, meaning "methods called during object creation or destruction".
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Remarks Finalizers cannot be defined in structs. They are only used with classes. A class can only have one finalizer. Finalizers cannot be inherited or overloaded. Finalizers cannot be called. They are invoked automatically. A finalizer does not take modifiers or have parameters.
For example, the following is a declaration of a finalizer for the Car class. Note Empty finalizers should not be used. When a class contains a finalizer, an entry is created in the Finalize queue.
When the finalizer is called, the garbage collector is invoked to process the queue. An empty finalizer just causes a needless loss of performance. The programmer has no control over when the finalizer is called because this is determined by the garbage collector. The garbage collector checks for objects that are no longer being used by the application. If it considers an object eligible for finalization, it calls the finalizer if any and reclaims the memory used to store the object.
NET Framework applications but not in. NET Core applications , finalizers are also called when the program exits. It is possible to force garbage collection by calling Collect , but most of the time, this should be avoided because it may create performance issues. Using finalizers to release resources In general, C does not require as much memory management as is needed when you develop with a language that does not target a runtime with garbage collection.
This is because the. NET Framework garbage collector implicitly manages the allocation and release of memory for your objects.
However, when your application encapsulates unmanaged resources such as windows, files, and network connections, you should use finalizers to free those resources. When the object is eligible for finalization, the garbage collector runs the Finalize method of the object. Explicit release of resources If your application is using an expensive external resource, we also recommend that you provide a way to explicitly release the resource before the garbage collector frees the object.
You do this by implementing a Dispose method from the IDisposable interface that performs the necessary cleanup for the object. This can considerably improve the performance of the application. Even with this explicit control over resources, the finalizer becomes a safeguard to clean up resources if the call to the Dispose method failed. For more details about cleaning up resources, see the following topics:.
Destructors, finalizers, and synchronization
Boehm Abstract We compare two different facilities for running cleanup actions for objects that are about to reach the end of their life. Theymake it easier to implement cleanup actions for objects of wellknown lifetime, especiallyin the presence of exceptions. We argue that these are fundamentallydifferent, and potentiallycomplementary, language facilities. We also tryto resolve some common misunderstandings about finalization in the process.
Finalizers (C# Programming Guide)