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Implementing Finalize and Dispose to Clean Up Unmanaged Resources

NoteNote

For information about finalizing and disposing resources using C++, see Destructors and Finalizers in Visual C++.

Class instances often encapsulate control over resources that are not managed by the runtime, such as window handles (HWND), database connections, and so on. Therefore, you should provide both an explicit and an implicit way to free those resources. Provide implicit control by implementing the protected Finalize on an object (destructor syntax in C# and C++). The garbage collector calls this method at some point after there are no longer any valid references to the object.

In some cases, you might want to provide programmers using an object with the ability to explicitly release these external resources before the garbage collector frees the object. If an external resource is scarce or expensive, better performance can be achieved if the programmer explicitly releases resources when they are no longer being used. To provide explicit control, implement the Dispose provided by the IDisposable. The consumer of the object should call this method when it is finished using the object. Dispose can be called even if other references to the object are alive.

Note that even when you provide explicit control using Dispose, you should provide implicit cleanup using the Finalize method. Finalize provides a backup to prevent resources from permanently leaking if the programmer fails to call Dispose.

For more information about implementing Finalize and Dispose to clean up unmanaged resources, see Garbage Collection. The following example illustrates the basic design pattern for implementing Dispose. This example requires the System namespace.


' Design pattern for a base class.

Public Class Base
   Implements IDisposable
    ' Field to handle multiple calls to Dispose gracefully.
    Dim disposed as Boolean = false

   ' Implement IDisposable.
   Public Overloads Sub Dispose() Implements IDisposable.Dispose
      Dispose(True)
      GC.SuppressFinalize(Me)
   End Sub

   Protected Overloads Overridable Sub Dispose(disposing As Boolean)
      If disposed = False Then
          If disposing Then
             ' Free other state (managed objects).
             disposed = True
          End If
          ' Free your own state (unmanaged objects).
          ' Set large fields to null.
      End If
   End Sub

   Protected Overrides Sub Finalize()
      ' Simply call Dispose(False).
      Dispose (False)
   End Sub
End Class

' Design pattern for a derived class.
Public Class Derived
   Inherits Base

    ' Field to handle multiple calls to Dispose gracefully.
    Dim disposed as Boolean = false

   Protected Overloads Overrides Sub Dispose(disposing As Boolean)
      If disposed = False Then
          If disposing Then
             ' Release managed resources.
          End If
          ' Release unmanaged resources.
          ' Set large fields to null.
          disposed = True
      End If
      ' Call Dispose on your base class.
      Mybase.Dispose(disposing)
   End Sub
   ' The derived class does not have a Finalize method
   ' or a Dispose method without parameters because it inherits
   ' them from the base class.
End Class


The following code expands the previous example to show the different ways Dispose is invoked and when Finalize is called. The stages of the disposing pattern are tracked with output to the console. The allocation and release of an unmanaged resource is handled in the derived class.


Imports System
Imports System.Collections.Generic
Imports System.Runtime.InteropServices

' Design pattern for a base class.
Public MustInherit Class Base
    Implements IDisposable

    Private disposed as Boolean = false
    Private instName As String
    Private trackingList As List(Of Object)

    Public Sub New(instanceName As String, tracking As List(Of Object))
        MyClass.instName = instanceName
        trackingList = tracking
        trackingList.Add(Me)
    End Sub

    Public ReadOnly Property InstanceName() As String
        Get
            Return instName
        End Get
    End Property

    'Implement IDisposable.
    Public Overloads Sub Dispose() Implements IDisposable.Dispose
        Console.WriteLine(vbNewLine + "[{0}].Base.Dispose()", instName)
        Dispose(true)
        GC.SuppressFinalize(Me)
    End Sub

    Protected Overloads Overridable Sub Dispose(disposing As Boolean)
        If disposed = False Then
            If disposing Then
                ' Free other state (managed objects).
                Console.WriteLine("[{0}].Base.Dispose(true)", instName)
                trackingList.Remove(Me)
                Console.WriteLine("[{0}] Removed from tracking list: {1:x16}",
                    instanceName, MyClass.GetHashCode())
            Else
                Console.WriteLine("[{0}].Base.Dispose(false)", instName)
            End If
            disposed = True
        End If
    End Sub

    Protected Overrides Sub Finalize()
        ' Simply call Dispose(False).
        Console.WriteLine(vbNewLine + "[{0}].Base.Finalize()", instName)
        Dispose(False)
    End Sub
End Class

' Design pattern for a derived class.
Public Class Derived
   Inherits Base

    Private disposed as Boolean = false
    Private umResource As IntPtr

    Public Sub New(instanceName As String, tracking As List(Of Object))
        MyBase.New(instanceName, tracking)
        ' Save the instance name as an unmanaged resource
        umResource = Marshal.StringToCoTaskMemAuto(instanceName)
    End Sub

    Protected Overloads Overrides Sub Dispose(disposing As Boolean)
        If disposed = False Then
            If disposing Then
                Console.WriteLine("[{0}].Derived.Dispose(true)", InstanceName)
                ' Release managed resources.
            Else
                Console.WriteLine("[{0}].Derived.Dispose(false)", InstanceName)
            End If
           ' Release unmanaged resources.
            If umResource <> IntPtr.Zero
                Marshal.FreeCoTaskMem(umResource)
                Console.WriteLine("[{0}] Unmanaged memory freed at {1:x16}", _
                    InstanceName, umResource.ToInt64())
                umResource = IntPtr.Zero
            End If
            disposed = True
        End If
        ' Call Dispose in the base class.
        MyBase.Dispose(disposing)
    End Sub
    ' The derived class does not have a Finalize method
    ' or a Dispose method without parameters because it inherits
    ' them from the base class.
End Class

Public Class TestDisposal
    Public Shared Sub Main()
        Dim tracking As New List(Of Object)()

        ' Dispose is not called, Finalize will be called later.
        Using Nothing
            Console.WriteLine(vbNewLine + "Disposal Scenario: #1" + vbNewLine)
            Dim d3 As New Derived("d1", tracking)
        End Using

        ' Dispose is implicitly called in the scope of the using statement.
        Using d1 As New Derived("d2", tracking)
            Console.WriteLine(vbNewLine + "Disposal Scenario: #2" + vbNewLine)
        End Using

        ' Dispose is explicitly called.
        Using Nothing
            Console.WriteLine(vbNewLine + "Disposal Scenario: #3" + vbNewLine)
            Dim d2 As New Derived("d3", tracking)
            d2.Dispose()
        End Using

        ' Again, Dispose is not called, Finalize will be called later.
        Using Nothing
            Console.WriteLine(vbNewLine + "Disposal Scenario: #4" + vbNewLine)
            Dim d4 As New Derived("d4", tracking)
        End Using

        ' List the objects remaining to dispose.
        Console.WriteLine(vbNewLine + "Objects remaining to dispose = {0:d}", tracking.Count)
        For Each dd As Derived in tracking
            Console.WriteLine("    Reference Object: {0:s}, {1:x16}",
                dd.InstanceName, dd.GetHashCode())
        Next dd
        ' Queued finalizers will be exeucted when Main() goes out of scope.
        Console.WriteLine(vbNewLine + "Dequeueing finalizers...")
    End Sub
End Class

' The program will display output similar to the following:
'
' Disposal Scenario: #1
'
'
' Disposal Scenario: #2
'
'
' [d2].Base.Dispose()
' [d2].Derived.Dispose(true)
' [d2] Unmanaged memory freed at 00000000001ce420
' [d2].Base.Dispose(true)
' [d2] Removed from tracking list: 0000000002bf8098
'
' Disposal Scenario: #3
'
'
' [d3].Base.Dispose()
' [d3].Derived.Dispose(true)
' [d3] Unmanaged memory freed at 00000000001ce420
' [d3].Base.Dispose(true)
' [d3] Removed from tracking list: 0000000000bb8560
'
' Disposal Scenario: #4
'
'
' Objects remaining to dispose = 2
'     Reference Object: d1, 000000000297b065
'     Reference Object: d4, 0000000003553390
'
' Dequeueing finalizers...
'
' [d4].Base.Finalize()
' [d4].Derived.Dispose(false)
' [d4] Unmanaged memory freed at 00000000001ce420
' [d4].Base.Dispose(false)
'
' [d1].Base.Finalize()
' [d1].Derived.Dispose(false)
' [d1] Unmanaged memory freed at 00000000001ce3f0
' [d1].Base.Dispose(false)


For an additional code example illustrating the design pattern for implementing Finalize and Dispose, see Implementing a Dispose Method.

Occasionally a domain-specific name is more appropriate than Dispose. For example, a file encapsulation might want to use the method name Close. In this case, implement Dispose privately and create a public Close method that calls Dispose. The following code example illustrates this pattern. You can replace Close with a method name appropriate to your domain. This example requires the System namespace.


' Do not make this method overridable.
' A derived class should not be allowed
' to override this method.
Public Sub Close()
   ' Call the Dispose method with no parameters.
   Dispose()
End Sub


The following rules outline the usage guidelines for the Finalize method:

  • Implement Finalize only on objects that require finalization. There are performance costs associated with Finalize methods.

  • If you require a Finalize method, consider implementing IDisposable to allow users of your class to avoid the cost of invoking the Finalize method.

  • Do not make the Finalize method more visible. It should be protected, not public.

  • An object's Finalize method should free any external resources that the object owns. Moreover, a Finalize method should release only resources that the object has held onto. The Finalize method should not reference any other objects.

  • Do not directly call a Finalize method on an object other than the object's base class. This is not a valid operation in the C# programming language.

  • Call the base class's Finalize method from an object's Finalize method.

    NoteNote

    The base class's Finalize method is called automatically with the C# and C++ destructor syntax.

The following rules outline the usage guidelines for the Dispose method:

  • Implement the dispose design pattern on a type that encapsulates resources that explicitly need to be freed. Users can free external resources by calling the public Dispose method.

  • Implement the dispose design pattern on a base type that commonly has derived types that hold onto resources, even if the base type does not. If the base type has a Close method, often this indicates the need to implement Dispose. In such cases, do not implement a Finalize method on the base type. Finalize should be implemented in any derived types that introduce resources that require cleanup.

  • Free any disposable resources a type owns in its Dispose method.

  • After Dispose has been called on an instance, prevent the Finalize method from running by calling the GC.SuppressFinalize. The exception to this rule is the rare situation in which work must be done in Finalize that is not covered by Dispose.

  • Call the base class's Dispose method if it implements IDisposable.

  • Do not assume that Dispose will be called. Unmanaged resources owned by a type should also be released in a Finalize method in the event that Dispose is not called.

  • Throw an ObjectDisposedException from instance methods on this type (other than Dispose) when resources are already disposed. This rule does not apply to the Dispose method because it should be callable multiple times without throwing an exception.

  • Propagate the calls to Dispose through the hierarchy of base types. The Dispose method should free all resources held by this object and any object owned by this object. For example, you can create an object such as a TextReader that holds onto a Stream and an Encoding, both of which are created by the TextReader without the user's knowledge. Furthermore, both the Stream and the Encoding can acquire external resources. When you call the Dispose method on the TextReader, it should in turn call Dispose on the Stream and the Encoding, causing them to release their external resources.

  • Consider not allowing an object to be usable after its Dispose method has been called. Re-creating an object that has already been disposed is a difficult pattern to implement.

  • Allow a Dispose method to be called more than once without throwing an exception. The method should do nothing after the first call.

Portions Copyright 2005 Microsoft Corporation. All rights reserved.

Portions Copyright Addison-Wesley Corporation. All rights reserved.

For more information on design guidelines, see the "Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries" book by Krzysztof Cwalina and Brad Abrams, published by Addison-Wesley, 2005.

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