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Object.Finalize Method

Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
'Declaration
Protected Overridable Sub Finalize

The Finalize 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

The Object class provides no implementation for the Finalize method, and the garbage collector does not mark types derived from Object for finalization unless they override the Finalize method.

If a type does override the Finalize 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 Finalize method automatically under the following conditions:

  • After the garbage collector has discovered that an object is 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.

Finalize 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.

Finalize operations have the following limitations:

  • The exact time when the finalizer executes 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 Finalize 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 Finalize or an override of Finalize 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 Finalize 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.

Important noteImportant

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 Finalize. For more information, see The SafeHandle alternative section.

The Object.Finalize method does nothing by default, but you should override Finalize only if necessary, and only to release unmanaged resources. Reclaiming memory tends to take much longer if a finalization operation runs, because it requires at least two garbage collections. In addition, you should override the Finalize method for reference types only. The common language runtime only finalizes reference types. It ignores finalizers on value types.

Every implementation of Finalize in a derived type must call its base type's implementation of Finalize. This is the only case in which application code is allowed to call Finalize.

NoteNote

The C# compiler does not allow you to override the Finalize 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 Finalize 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 Finalize method to free unmanaged resources in the event that the Dispose method is not called.

Finalize 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; Finalize cannot be called on resurrected objects during garbage collection. There is one action that your implementation of Finalize 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:

The following example uses the dispose pattern with safe handles instead of overriding the Finalize 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.

Imports Microsoft.Win32.SafeHandles
Imports System
Imports System.ComponentModel
Imports System.IO
Imports System.Runtime.InteropServices
Imports System.Text

Public Class FileAssociationInfo : Implements IDisposable
   ' Private variables. 
   Private ext As String 
   Private openCmd As String 
   Private args As String 
   Private hExtHandle, hAppIdHandle As SafeRegistryHandle

   ' Windows API calls. 
   Private Declare Unicode Function RegOpenKeyEx Lib"advapi32.dll" _
                   Alias "RegOpenKeyExW" (hKey As IntPtr, lpSubKey As String, _
                   ulOptions As Integer, samDesired As Integer, _
                   ByRef phkResult As IntPtr) As Integer 
   Private Declare Unicode Function RegQueryValueEx Lib "advapi32.dll" _
                   Alias "RegQueryValueExW" (hKey As IntPtr, _
                   lpValueName As String, lpReserved As Integer, _
                   ByRef lpType As UInteger, lpData As String, _
                   ByRef lpcbData As UInteger) As Integer    
   Private Declare Function RegSetValueEx Lib "advapi32.dll" _
                  (hKey As IntPtr, _
                  <MarshalAs(UnmanagedType.LPStr)> lpValueName As String, _
                  reserved As Integer, dwType As UInteger, _
                  <MarshalAs(UnmanagedType.LPStr)> lpData As String, _
                  cpData As Integer) As Integer  
   Private Declare Function RegCloseKey Lib "advapi32.dll" _
                  (hKey As IntPtr) As Integer 

   ' Windows API constants. 
   Private Const HKEY_CLASSES_ROOT As Integer = &h80000000
   Private Const ERROR_SUCCESS As Integer = 0

   Private Const KEY_QUERY_VALUE As Integer = 1
   Private Const KEY_SET_VALUE As Integer = &h2

   Private REG_SZ As UInteger = 1

   Private Const MAX_PATH As Integer  = 260

   Public Sub New(fileExtension As String)
      Dim retVal As Integer = 0
      Dim lpType As UInteger = 0

      If Not fileExtension.StartsWith(".") Then 
         fileExtension = "." + fileExtension
      End If   
      ext = fileExtension

      Dim hExtension As IntPtr = IntPtr.Zero
      ' Get the file extension value.
      retVal = RegOpenKeyEx(New IntPtr(HKEY_CLASSES_ROOT), fileExtension, 0, 
                            KEY_QUERY_VALUE, hExtension)
      if retVal <> ERROR_SUCCESS Then  
         Throw New Win32Exception(retVal)
      End If   
      ' Instantiate the first SafeRegistryHandle.
      hExtHandle = New SafeRegistryHandle(hExtension, True)

      Dim appId As New String(" "c, MAX_PATH)
      Dim appIdLength As UInteger = CUInt(appId.Length)
      retVal = RegQueryValueEx(hExtHandle.DangerousGetHandle(), String.Empty, _
                               0, lpType, appId, appIdLength)
      if retVal <> ERROR_SUCCESS Then 
         Throw New Win32Exception(retVal)
      End If    
      ' We no longer need the hExtension handle.
      hExtHandle.Dispose()

      ' Determine the number of characters without the terminating null.
      appId = appId.Substring(0, CInt(appIdLength) \ 2 - 1) + "\shell\open\Command" 

      ' Open the application identifier key. 
      Dim exeName As New string(" "c, MAX_PATH)
      Dim exeNameLength As UInteger = CUInt(exeName.Length)
      Dim hAppId As IntPtr
      retVal = RegOpenKeyEx(New IntPtr(HKEY_CLASSES_ROOT), appId, 0, 
                            KEY_QUERY_VALUE Or KEY_SET_VALUE, hAppId)
      If retVal <> ERROR_SUCCESS Then  
         Throw New Win32Exception(retVal)
      End If    

      ' Instantiate the second SafeRegistryHandle.
      hAppIdHandle = New SafeRegistryHandle(hAppId, True)

      ' Get the executable name for this file type. 
      Dim exePath As New string(" "c, MAX_PATH)
      Dim exePathLength As UInteger = CUInt(exePath.Length)
      retVal = RegQueryValueEx(hAppIdHandle.DangerousGetHandle(), _
                               String.Empty, 0, lpType, exePath, exePathLength)
      If retVal <> ERROR_SUCCESS Then 
         Throw New Win32Exception(retVal)
      End If      
      ' Determine the number of characters without the terminating null.
      exePath = exePath.Substring(0, CInt(exePathLength) \ 2 - 1)

      exePath = Environment.ExpandEnvironmentVariables(exePath)
      Dim position As Integer = exePath.IndexOf("%"c)
      If position >= 0 Then
         args = exePath.Substring(position)
         ' Remove command line parameters ('%0', etc.).
         exePath = exePath.Substring(0, position).Trim()
      End If   
      openCmd = exePath
   End Sub 

   Public ReadOnly Property Extension As String 
      Get 
         Return ext
      End Get 
   End Property 

   Public Property Open As String 
      Get 
         Return openCmd
      End Get     
      Set  
        If hAppIdHandle.IsInvalid Or hAppIdHandle.IsClosed Then 
           Throw New InvalidOperationException("Cannot write to registry key.")
        End If     
        If Not File.Exists(value) Then 
           Dim message As String = String.Format("'{0}' does not exist", value)
           Throw New FileNotFoundException(message) 
        End If 
        Dim cmd As String = value + " %1" 
        Dim retVal As Integer = RegSetValueEx(hAppIdHandle.DangerousGetHandle(), String.Empty, 0, 
                                              REG_SZ, value, value.Length + 1)
        If retVal <> ERROR_SUCCESS Then  
           Throw New Win32Exception(retVal)
        End If                              
      End Set 
   End Property    

   Public Sub Dispose() _
      Implements IDisposable.Dispose 
      Dispose(true)
      GC.SuppressFinalize(Me)
   End Sub    

   Protected Sub Dispose(disposing As Boolean)
      ' Ordinarily, we release unmanaged resources here  
      ' but all are wrapped by safe handles. 

      ' Release disposable objects. 
      If disposing Then 
         If hExtHandle IsNot Nothing Then hExtHandle.Dispose()
         If hAppIdHandle IsNot Nothing Then hAppIdHandle.Dispose()
      End If 
   End Sub 
End Class

The following example verifies that the Finalize method is called when an object that overrides Finalize is destroyed. Note that, in a production application, the Finalize 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 Finalize method.

Imports System.Diagnostics

Public Class ExampleClass
   Dim sw As StopWatch

   Public Sub New()
      sw = Stopwatch.StartNew()
      Console.WriteLine("Instantiated object")
   End Sub  

   Public Sub ShowDuration()
      Console.WriteLine("This instance of {0} has been in existence for {1}",
                        Me, sw.Elapsed)
   End Sub 

   Protected Overrides Sub Finalize()
      Console.WriteLine("Finalizing object")
      sw.Stop()
      Console.WriteLine("This instance of {0} has been in existence for {1}",
                        Me, sw.Elapsed)
   End Sub 
End Class 

Module Demo
   Public Sub Main()
      Dim ex As New ExampleClass()
      ex.ShowDuration()
   End Sub 
End Module 
' The example displays output like the following: 
'    Instantiated object 
'    This instance of ExampleClass has been in existence for 00:00:00.0011060 
'    Finalizing object 
'    This instance of ExampleClass has been in existence for 00:00:00.0036294

For an additional example that overrides the Finalize method, see the GC.SuppressFinalize method.

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, 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.

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