Object.Finalize Method ()

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.

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
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.

Note

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.

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.

• SafeNCryptKeyHandle, SafeNCryptProviderHandle, and SafeNCryptSecretHandle are wrapper classes for cryptographic handles.

• 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 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.

using Microsoft.Win32.SafeHandles;
using System;
using System.ComponentModel;
using System.IO;
using System.Runtime.InteropServices;

public class FileAssociationInfo : IDisposable
{
   // Private variables.
   private String ext;
   private String openCmd;
   private String args;
   private SafeRegistryHandle hExtHandle, hAppIdHandle;

   // Windows API calls.
   [DllImport("advapi32.dll", CharSet= CharSet.Auto, SetLastError=true)]
   private static extern int RegOpenKeyEx(IntPtr hKey, 
                  String lpSubKey, int ulOptions, int samDesired,
                  out IntPtr phkResult);
   [DllImport("advapi32.dll", CharSet= CharSet.Unicode, EntryPoint = "RegQueryValueExW",
              SetLastError=true)]
   private static extern int RegQueryValueEx(IntPtr hKey,
                  string lpValueName, int lpReserved, out uint lpType, 
                  string lpData, ref uint lpcbData);   
   [DllImport("advapi32.dll", SetLastError = true)]
   private static extern int RegSetValueEx(IntPtr hKey, [MarshalAs(UnmanagedType.LPStr)] string lpValueName,
                  int Reserved, uint dwType, [MarshalAs(UnmanagedType.LPStr)] string lpData,
                  int cpData);
   [DllImport("advapi32.dll", SetLastError=true)]
   private static extern int RegCloseKey(UIntPtr hKey);

   // Windows API constants.
   private const int HKEY_CLASSES_ROOT = unchecked((int) 0x80000000);
   private const int ERROR_SUCCESS = 0;

    private const int KEY_QUERY_VALUE = 1;
    private const int KEY_SET_VALUE = 0x2;

   private const uint REG_SZ = 1;

   private const int MAX_PATH = 260;

   public FileAssociationInfo(String fileExtension)
   {
      int retVal = 0;
      uint lpType = 0;

      if (!fileExtension.StartsWith("."))
             fileExtension = "." + fileExtension;
      ext = fileExtension;

      IntPtr hExtension = IntPtr.Zero;
      // Get the file extension value.
      retVal = RegOpenKeyEx(new IntPtr(HKEY_CLASSES_ROOT), fileExtension, 0, KEY_QUERY_VALUE, out hExtension);
      if (retVal != ERROR_SUCCESS) 
         throw new Win32Exception(retVal);
      // Instantiate the first SafeRegistryHandle.
      hExtHandle = new SafeRegistryHandle(hExtension, true);

      string appId = new string(' ', MAX_PATH);
      uint appIdLength = (uint) appId.Length;
      retVal = RegQueryValueEx(hExtHandle.DangerousGetHandle(), String.Empty, 0, out lpType, appId, ref appIdLength);
      if (retVal != ERROR_SUCCESS)
         throw new Win32Exception(retVal);
      // We no longer need the hExtension handle.
      hExtHandle.Dispose();

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

      // Open the application identifier key.
      string exeName = new string(' ', MAX_PATH);
      uint exeNameLength = (uint) exeName.Length;
      IntPtr hAppId;
      retVal = RegOpenKeyEx(new IntPtr(HKEY_CLASSES_ROOT), appId, 0, KEY_QUERY_VALUE | KEY_SET_VALUE,
                            out hAppId);
       if (retVal != ERROR_SUCCESS) 
         throw new Win32Exception(retVal);

      // Instantiate the second SafeRegistryHandle.
      hAppIdHandle = new SafeRegistryHandle(hAppId, true);

      // Get the executable name for this file type.
      string exePath = new string(' ', MAX_PATH);
      uint exePathLength = (uint) exePath.Length;
      retVal = RegQueryValueEx(hAppIdHandle.DangerousGetHandle(), String.Empty, 0, out lpType, exePath, ref exePathLength);
      if (retVal != ERROR_SUCCESS)
         throw new Win32Exception(retVal);

      // Determine the number of characters without the terminating null.
      exePath = exePath.Substring(0, (int) exePathLength / 2 - 1);
      // Remove any environment strings.
      exePath = Environment.ExpandEnvironmentVariables(exePath);

      int position = exePath.IndexOf('%');
      if (position >= 0) {
         args = exePath.Substring(position);
         // Remove command line parameters ('%0', etc.).
         exePath = exePath.Substring(0, position).Trim();
      }
      openCmd = exePath;   
   }

   public String Extension
   { get { return ext; } }

   public String Open
   { get { return openCmd; } 
     set {
        if (hAppIdHandle.IsInvalid | hAppIdHandle.IsClosed)
           throw new InvalidOperationException("Cannot write to registry key."); 
        if (! File.Exists(value)) {
           string message = String.Format("'{0}' does not exist", value);
           throw new FileNotFoundException(message); 
        }
        string cmd = value + " %1";
        int retVal = RegSetValueEx(hAppIdHandle.DangerousGetHandle(), String.Empty, 0, 
                                   REG_SZ, value, value.Length + 1);
        if (retVal != ERROR_SUCCESS)
           throw new Win32Exception(retVal);                          
     } }

   public void Dispose() 
   {
      Dispose(true);
      GC.SuppressFinalize(this);
   }   

   protected void Dispose(bool disposing)
   {
      // Ordinarily, we release unmanaged resources here; 
      // but all are wrapped by safe handles.

      // Release disposable objects.
      if (disposing) {
         if (hExtHandle != null) hExtHandle.Dispose();
         if (hAppIdHandle != null) hAppIdHandle.Dispose();
      }
   }
}