Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.
Assembly: mscorlib (in mscorlib.dll)
The method is used to perform cleanup operations on unmanaged resources held by the current object before the object is destroyed. The method is protected and therefore is accessible only through this class or through a derived class.
In this section:
How finalization works
If a type does override the method, the garbage collector adds an entry for each instance of the type to an internal structure called the finalization queue. The finalization queue contains entries for all the objects in the managed heap whose finalization code must run before the garbage collector can reclaim their memory. The garbage collector then calls the method automatically under the following conditions:
After an object becomes inaccessible, unless the object has been exempted from finalization by a call to the GC.SuppressFinalize method.
During shutdown of an application domain, unless the object is exempt from finalization. During shutdown, even objects that are still accessible are finalized.
is automatically called only once on a given instance, unless the object is re-registered by using a mechanism such as GC.ReRegisterForFinalize and the GC.SuppressFinalize method has not been subsequently called.
operations have the following limitations:
The exact time when the finalizer executes during garbage collection is undefined. To ensure deterministic release of resources for instances of your class, implement a Close method or provide a IDisposable.Dispose implementation.
The finalizers of two objects are not guaranteed to run in any specific order, even if one object refers to the other. That is, if Object A has a reference to Object B and both have finalizers, Object B might have already been finalized when the finalizer of Object A starts.
The thread on which the finalizer runs is unspecified.
The method might not run to completion or might not run at all under the following exceptional circumstances:
If another finalizer blocks indefinitely (goes into an infinite loop, tries to obtain a lock it can never obtain, and so on). Because the runtime tries to run finalizers to completion, other finalizers might not be called if a finalizer blocks indefinitely.
If the process terminates without giving the runtime a chance to clean up. In this case, the runtime's first notification of process termination is a DLL_PROCESS_DETACH notification.
The runtime continues to finalize objects during shutdown only while the number of finalizable objects continues to decrease.
If or an override of throws an exception, and the runtime is not hosted by an application that overrides the default policy, the runtime terminates the process and no active try/finally blocks or finalizers are executed. This behavior ensures process integrity if the finalizer cannot free or destroy resources.
Notes for implementers
You should override for a class that uses unmanaged resources such as file handles or database connections that must be released when the managed object that uses them is discarded during garbage collection.
If a SafeHandle object is available that wraps your unmanaged resource, the recommended alternative is to implement the dispose pattern with a safe handle and not override . For more information, see The SafeHandle alternative section.
The method does nothing by default, but you should override only if necessary, and only to release unmanaged resources. Reclaiming memory during garbage collection tends to take much longer if a finalization operation runs, because it requires at least two garbage collections. In addition, you should override the method for reference types only. The common language runtime only finalizes reference types. It ignores finalizers on value types.
Every implementation of in a derived type must call its base type's implementation of . This is the only case in which application code is allowed to call .
The C# compiler does not allow you to override the method. Instead, you provide a finalizer by implementing a destructor for your class. A C# destructor automatically calls the destructor of its base class.
Visual C++ also provides its own syntax for implementing the method. For more information, see the "Destructors and finalizers" section of How to: Define and Consume Classes and Structs (C++/CLI).
Because garbage collection is non-deterministic, you do not know precisely when the garbage collector performs finalization. To release resources immediately, you can also choose to implement the dispose pattern and the IDisposable interface. The IDisposable.Dispose implementation can be called by consumers of your class to free unmanaged resources, and you can use the method to free unmanaged resources in the event that the Dispose method is not called.
can take almost any action, including resurrecting an object (that is, making the object accessible again) after it has been cleaned up during garbage collection. However, the object can only be resurrected once; cannot be called on resurrected objects during garbage collection. There is one action that your implementation of should never take: it should never throw an exception.
The SafeHandle alternative
Creating reliable finalizers is often difficult, because you cannot make assumptions about the state of your application, and because unhandled system exceptions such as OutOfMemoryException and StackOverflowException terminate the finalizer. Instead of implementing a finalizer for your class to release unmanaged resources, you can use an object that is derived from the System.Runtime.InteropServices.SafeHandle class to wrap your unmanaged resources, and then implement the dispose pattern without a finalizer. The .NET Framework provides the following classes in the Microsoft.Win32 namespace that are derived from System.Runtime.InteropServices.SafeHandle:
SafeFileHandle is a wrapper class for a file handle.
SafeMemoryMappedFileHandle is a wrapper class for memory-mapped file handles.
SafeMemoryMappedViewHandle is a wrapper class for a pointer to a block of unmanaged memory.
SafePipeHandle is a wrapper class for pipe handles.
SafeRegistryHandle is a wrapper class for a handle to a registry key.
SafeWaitHandle is a wrapper class for a wait handle.
The following example uses the dispose pattern with safe handles instead of overriding the method. It defines a FileAssociation class that wraps registry information about the application that handles files with a particular file extension. The two registry handles returned as out parameters by Windows RegOpenKeyEx function calls are passed to the SafeRegistryHandle constructor. The type's protected Dispose method then calls the SafeRegistryHandle.Dispose method to free these two handles.
The following example verifies that the method is called when an object that overrides is destroyed. Note that, in a production application, the method would be overridden to release unmanaged resources held by the object. Also note that the C# example provides a destructor instead of overriding the method.
For an additional example that overrides the method, see the GC.SuppressFinalize method.
Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.