Home > Articles > Programming > Windows Programming

  • Print
  • + Share This
Like this article? We recommend

Dispose and Finalize

A second major design pattern that you'll find in the .NET Framework is that of dispose and finalize. Basically, you'll find this pattern in classes that acquire and hold on to resources such as file and window handles or network and database connections. This is the case because the Common Language Runtime (CLR) within which your .NET code runs implements nondeterministic finalization, meaning that the CLR ultimately decides when object instances are no longer able to be referenced and when those instances are deallocated in memory. The process of reclaiming memory by deallocating instances is referred to as garbage collection, or simply GC. This approach is obviously much different than the deterministic techniques used in previous versions of VB (in which setting an object to Nothing deallocated the object immediately and caused its Terminate event to run) or C++ (in which an object could be deallocated at any time, causing its destructor to run). Obviously, objects that hold on to resources such as file handles or database connections need a way to release those resources in a timely fashion—hence the dispose and finalize design pattern.

The idea behind this pattern is that the client decides when the resources are no longer required and calls the Dispose or Close method on the class. This method then cleans up the resources and ensures that the object can no longer be used. However, adding a Dispose or Close method is not sufficient in and of itself because, if the client forgets to call it, the resource may not then be released, causing a leak in your application. To ensure that resources are cleaned up, your classes can implement a destructor that gets called automatically by the CLR when the GC process finally cleans up the instance. As a result, in your classes you'll want to implement a Dispose or Close method along with a destructor, as shown in Listing 2.

Listing 2: Implementing the Dispose and Finalize Pattern

Public MustInherit Class QuilogyDataAccess : Implements IDisposable

 Protected _disposed As Boolean = False
 
 Protected Overridable Sub Dispose(ByVal disposing As Boolean)
  If disposing Then
   ' Call dispose on any objects referenced by this object
  End If
  ' Release unmanaged resources
 End Sub

 Protected Overridable Sub Dispose() Implements IDisposable.Dispose
   Dispose(True)
   _disposed = True
  ' Take off finalization queue
  GC.SuppressFinalize(Me)
 End Sub

 Protected Overrides Sub Finalize()  
  Me.Dispose(False)
 End Sub

End Class


Public Class Customers : Inherits QuilogyDataAccess

 Public Sub Close()
  Me.Dispose(True)
 End Sub

 Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
  If disposing Then
   ' Call dispose on any objects referenced by this object
  End If
  ' Release unmanaged resources

  MyBase.Dispose(disposing)
 End Sub

 Public Sub GetCustomers()
  ' Make sure the object has not been disposed
  If MyBase._disposed Then
   Throw New ObjectDisposedException("Customers", "Has been closed")
  End If

  ' Do other work here
 End Sub

End Class

In Listing 2, you'll notice that the QuilogyDataAccess class is a base (MustInherit, abstract in C#) class that ostensibly creates and holds on to a managed resource, such as a database connection, or an unmanaged resource, such as a file or window handle. As a result, this class exposes a Dispose method by implementing the IDisposable interface and includes a destructor implemented by overriding the Finalize method.

You'll notice that when the Dispose method inherited from the IDisposable interface is called, it calls a second Dispose method, passing it True. This second Dispose method cleans up the managed resources by calling their Dispose methods if passed True, and then proceeds to deallocate unmanaged resources. When it is finished, it returns and the original Dispose method then sets the protected disposed flag to True, to indicate that the class has already been disposed of. It then calls the static SuppressFinalize method of the GC class. This is done to prevent the Finalize method (the destructor) from running when the GC process next runs. Calling SuppressFinalize is simply an optimization that allows the instance to ultimately be deallocated sooner because all objects with a destructor are placed in a special queue that must first run their destructors before deallocating them.

Although it may seem strange that you need to make this differentiation, it is important because, if the GC process ends up running the Finalize method, you have no way of knowing whether the managed objects referenced by the class have already been finalized. This is the case because the order of finalization is not deterministic, regardless of the order in which the objects were deemed unreachable by the GC. As a result, you wouldn't want to call their Dispose methods if they had already been deallocated. You'll notice that the Finalize method simply executes the Dispose method, passing in False to ensure that unmanaged resources are cleaned up if the Dispose method is never executed and the Finalize method is being run by the GC process. Incidentally, the pattern shown in Listing 1 works with C# as well where the destructor is identified using a ~, as in this example:

~ QuilogyDataAccess()
{
 this.Dispose(false);
}

The other difference between C# and VB .NET is that destructors in C# can be called only by the GC process, not directly by client code; the Finalize method in VB .NET can be called by other code in the class or its descendants. This is why the destructor calls Dispose and not vice versa.

The Customers class is then derived from QuilogyDataAccess and can override the Dispose method to clean up its own resources by calling Dispose on any objects that it references before calling the Dispose method of the base class. As with the base class, the Dispose method of the derived class accepts an argument that determines whether managed resources are cleaned up in addition to unmanaged resources. By passing True into the method, as is done by the Close method, the class calls the Dispose methods of any managed objects that it references, in addition to deallocating unmanaged resources such as file and window handles.

You'll notice that the Customers class also exposes a public Close method that allows clients to clean up the resources explicitly. The Close method simply calls the Dispose method to do the work and can be used when the Close method makes more sense semantically. The GetCustomers method of the Customers class shows how a method can then check to see whether the object has previously been disposed of before allowing execution to continue. If the object has already been disposed of, you can throw the ObjectDisposedException. Note that if you want to re-create the resources if a method such as GetCustomers is called after the object has been disposed, you would also need to call the ReRegisterForFinalize method of the GC class to make sure that the instance will be finalized by the GC eventually if Close or Dispose is not called.

Clients using the Customers class can then instantiate the class and work with it as normal, making sure to call the Close method when finished or casting to the IDisposable interface like this:

Dim c As New Customers()

' Do other work here
c.GetCustomers()
c.Close()

Or

Dim c As New Customers()
Dim d As IDisposable

' Do other work here
c.GetCustomers()

d = c
d.Dispose()

Alternatively, the C# language includes the using statement that automatically casts to the IDisposable interface and calls the Dispose method when the block is exited, like this:

using (Customers c = new Customers()) {
 // Do other work here
 c.GetCustomers();
}

It should be mentioned that the compiler will allow the using statement only when the class to which it refers implements the IDisposable interface.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020