In object-oriented programming languages that use garbage collection, a finalizer is a special method that is executed when an object is garbage collected. It is similar in function to a destructor. In less technical terms, a finalizer is a piece of code that ensures that certain necessary actions are taken when an acquired resource (such as a file or access to a hardware device) is no longer being used. This could be closing the file or signalling to the operating system that the hardware device is no longer needed. However, as noted below, finalizers are not the preferred way to accomplish this and for the most part serve as a fail-safe.

Unlike destructors, finalizers are not deterministic. A destructor is run when the program explicitly frees an object. A finalizer, by contrast, is executed when the internal garbage collection system frees the object.

Programming languages which use finalizers include Java and C#. In C#, and a few others which support finalizers, the syntax for declaring a finalizer mimics that of destructors in C++.

Due to the lack of programmer control over their execution, it is usually recommended to avoid finalizers for any but the most trivial operations. In particular, operations often performed in destructors are not usually appropriate for finalizers. For example, destructors are often used to free expensive resources such as file or network handles. If placed in a finalizer, the resources may remain in use for long periods of time after the program is finished with them. Instead, most languages encourage a "dispose" design pattern whereby the object has a method to clean up the object's resources, leaving the finalizer as a fail-safe in the case where the dispose method doesn't get called. (The C# language supports the dispose pattern implicitly, via the IDisposable interface and the using keyword.)

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