Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
Assembly: mscorlib (in mscorlib.dll)
Use this method to close or release unmanaged resources such as files, streams, and handles held by an instance of the class that implements this interface. By convention, this method is used for all tasks associated with freeing resources held by an object, or preparing an object for reuse.
If you are using a class that implements the IDisposable interface, you should call its implementation when you are finished using the class. For more information, see the "Using an object that implements IDisposable" section in the IDisposable topic.
When implementing this method, ensure that all held resources are freed by propagating the call through the containment hierarchy. For example, if an object A allocates an object B, and object B allocates an object C, then A's implementation must call on B, which must in turn call on C.
C++ programmers should read Destructors and Finalizers in Visual C++. The C++ compiler supports deterministic disposal of resources and does not allow direct implementation of the method.
An object must also call the method of its base class if the base class implements IDisposable. For more information about implementing IDisposable on a base class and its subclasses, see the "IDisposable and the inheritance hierarchy" section in the IDisposable topic.
If an object's method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its method is called multiple times. Instance methods other than can throw an ObjectDisposedException when resources are already disposed.
Users might expect a resource type to use a particular convention to denote an allocated state versus a freed state. An example of this is stream classes, which are traditionally thought of as open or closed. The implementer of a class that has such a convention might choose to implement a public method with a customized name, such as Close, that calls the method.
Because the method must be called explicitly, there is always a danger that the unmanaged resources will not be released, because the consumer of an object fails to call its method. There are two ways to avoid this:
Wrap the managed resource in an object derived from System.Runtime.InteropServices.SafeHandle. Your implementation then calls the method of the System.Runtime.InteropServices.SafeHandle instances. For more information, see "The SafeHandle alternative" section in the Object.Finalize topic.
Implement a finalizer to free resources when is not called. By default, the garbage collector automatically calls an object's finalizer before reclaiming its memory. However, if the method has been called, it is typically unnecessary for the garbage collector to call the disposed object's finalizer. To prevent automatic finalization, implementations can call the GC.SuppressFinalize method.
When you use an object that accesses unmanaged resources, such as a StreamWriter, a good practice is to create the instance with a using statement. The using statement automatically closes the stream and calls on the object when the code that is using it has completed. For an example, see the StreamWriter class.
The following example shows how you can implement the method.
Imports System Imports System.ComponentModel ' The following example demonstrates how to create ' a resource class that implements the IDisposable interface ' and the IDisposable.Dispose method. Public Class DisposeExample ' A class that implements IDisposable. ' By implementing IDisposable, you are announcing that ' instances of this type allocate scarce resources. Public Class MyResource Implements IDisposable ' Pointer to an external unmanaged resource. Private handle As IntPtr ' Other managed resource this class uses. Private component As component ' Track whether Dispose has been called. Private disposed As Boolean = False ' The class constructor. Public Sub New(ByVal handle As IntPtr) Me.handle = handle End Sub ' Implement IDisposable. ' Do not make this method virtual. ' A derived class should not be able to override this method. Public Overloads Sub Dispose() Implements IDisposable.Dispose Dispose(True) ' This object will be cleaned up by the Dispose method. ' Therefore, you should call GC.SupressFinalize to ' take this object off the finalization queue ' and prevent finalization code for this object ' from executing a second time. GC.SuppressFinalize(Me) End Sub ' Dispose(bool disposing) executes in two distinct scenarios. ' If disposing equals true, the method has been called directly ' or indirectly by a user's code. Managed and unmanaged resources ' can be disposed. ' If disposing equals false, the method has been called by the ' runtime from inside the finalizer and you should not reference ' other objects. Only unmanaged resources can be disposed. Protected Overridable Overloads Sub Dispose(ByVal disposing As Boolean) ' Check to see if Dispose has already been called. If Not Me.disposed Then ' If disposing equals true, dispose all managed ' and unmanaged resources. If disposing Then ' Dispose managed resources. component.Dispose() End If ' Call the appropriate methods to clean up ' unmanaged resources here. ' If disposing is false, ' only the following code is executed. CloseHandle(handle) handle = IntPtr.Zero ' Note disposing has been done. disposed = True End If End Sub ' Use interop to call the method necessary ' to clean up the unmanaged resource. <System.Runtime.InteropServices.DllImport("Kernel32")> _ Private Shared Function CloseHandle(ByVal handle As IntPtr) As [Boolean] End Function ' This finalizer will run only if the Dispose method ' does not get called. ' It gives your base class the opportunity to finalize. ' Do not provide finalize methods in types derived from this class. Protected Overrides Sub Finalize() ' Do not re-create Dispose clean-up code here. ' Calling Dispose(false) is optimal in terms of ' readability and maintainability. Dispose(False) MyBase.Finalize() End Sub End Class Public Shared Sub Main() ' Insert code here to create ' and use the MyResource object. End Sub End Class