Home > Articles > Programming > Windows Programming

Inheritance and VB.NET

  • Print
  • + Share This
Inheritance is often considered one of the most exciting features of Visual Basic.NET - a feature that is considered fundamental to the creation of object-based systems, and one that has been missing from VB until this version. Although somehow I managed to build systems without inheritance before .NET arrived, the addition of inheritance to the Visual Basic language is an important feature worth discussing.
This article is excerpted from Chapter 7, "Working with Objects," of Sams Teach Yourself VB.NET in 21 Days.
Like this article? We recommend

This article assumes some programming knowledge, the ability to work within an Integrated Development Environment and some familiarity with object-oriented technologies. This material is based on Beta 2 release version of Microsoft's .NET technology.

To some people, Inheritance is one of the most exciting features of Visual Basic.NET - a feature that is considered fundamental to the creation of object-based systems and that has been missing from VB until this version. I am not going to argue with this opinion, but somehow I managed to build systems for many years before .NET arrived, without inheritance. Regardless, the addition of inheritance to the Visual Basic language is an important feature and worth a little discussion.

Objects are a way to represent concepts or entities in code, and each of the object features of Visual Basic is designed to help you make the most useful representation possible. In many situations, one entity or concept is really what we would call a subobject of a more basic entity or concept. Many examples of this are used in programming books, and sadly I am not going to come up with anything too revolutionary. Consider our class of objects designed to represent cars, such as a Ford Mustang, Toyota Celica, or Chevy Cavalier. The class would have various properties such as Doors (number of doors the car has), MaxSpeed, Color, and others.

This general Car class really contains several subclasses of objects, such as Hatchbacks and Convertibles. Those classes would, of course, have all the properties of their parent class, Car, such as Doors, MaxSpeed, and Color, but they could also have unique properties of their own. A hatchback could have properties that described the size and behavior of its rear door. This relationship, between Car and its subclasses of Hatchback and Convertible, would be considered to be a parent-child relationship, and the method for representing this relationship in our systems is called inheritance. The class Hatchback is said to inherit from its base class Car. This relationship means that, in addition to any properties and methods created in the child class, the child will also possess all the properties and methods of the parent.

The previous example started with our Car class and headed downwards. Let's take the same starting point of Car and head upwards for a more detailed example of inheritance. To start, we could have a base class of Vehicle, which could represent any type of vehicle (boat, car, truck, plane) and has the properties of MaxSpeed, NumberOfPassengers, Color, and Description. We could easily represent this class in Visual Basic code, as shown in Listing 7.1.

Listing 7.1 Our Vehicle Class

  Public Class Vehicle

    Public Property MaxSpeed() As Long
      Get

      End Get
      Set

      End Set
    End Property

    Public Property NumberOfPassengers() As Long
      Get

      End Get
      Set

      End Set
    End Property

    Public Property Color() As String
      Get

      End Get
      Set

      End Set
    End Property

    Public Function Description() As String

    End Function
  End Class

The code that goes within the various procedures in that class is not really relevant to our example, so we will just leave it blank for now. If we were to jump to some code to try using our object (for instance, in the Sub Main() section), we would see that we can create objects of type Vehicle and work with their properties, as in Listing 7.2.

Listing 7.2 Working with Our Vehicle Class

  Module Main

    Sub Main()
      Dim objVehicle As Vehicle
      objVehicle = New Vehicle

      objVehicle.Color = "Red"
      objVehicle.MaxSpeed = 100

    End Sub

  End Module

Now, by adding an additional class to our project, we can create a class, Car, that inherits from Vehicle, just like the real class of object Car is a subclass or child of the Vehicle class of objects. Because we are creating a class designed to deal with just cars, we can add a couple of properties (NumberOfDoors and NumberOfTires) specific to this subclass of Vehicle as shown in Listing 7.3.

Listing 7.3 Working with Our Vehicle Class

  Public Class Car
    Inherits Vehicle

    Public Property NumberOfTires() As Long
      Get

      End Get
      Set

      End Set
    End Property

    Public Property NumberOfDoors() As Long
      Get

      End Get
      Set

      End Set
    End Property

  End Class

The key to this code is the line Inherits Vehicle, which tells Visual Basic that this class is a child of the Vehicle class and therefore should inherit of all that class's properties and methods. Once again, no code is actually placed into any of these property definitions because they are not really relevant at this time. After that code is in place, without having to do anything else, we can see the effect of the Inherits statement.

Returning to our Main() procedure, we can create an object of type Car, and we'll quickly see that it exposes both its own properties and those of the parent class.

When an inherited class adds new methods or properties, it is said to be extending the base class. In addition to extending, it is also possible for a child class to override some or all of the functionality of the base class. This is done when the child implements a method or property that is also defined in the parent or base class. In such a case, the code in the child will be executed instead of the code in the parent, allowing you to create specialized versions of the base method or property.

For a child class to override some part of the base class, that portion must be marked Overridable in the base class definition. For instance, in the version of Vehicle listed earlier, none of its properties had the keyword Overridable and therefore child classes would be unable to provide their own implementations. As an example of how overriding is set up in the base and child classes, the code in Listing 7.4 marks the Description() function as being overridable and then overrides it in the Car child class. Note that non-relevant portions of the two classes have been removed for clarity.

Listing 7.4 Using the Overridable and Overrides Keywords

  Public Class Vehicle

    'Code removed for simplicity....

    Overridable Public Function Description() As String
      Return "This is my generic vehicle description!"
    End Function
  End Class

  Public Class Car
    Inherits Vehicle

    'Code removed for simplicity....

    Overrides Public Function Description() As String
      Return "This is my Car Description"

    End Function

  End Class

When overriding a method or property, as we did in Listing 7.4, you can refer back to the original member of the base class by using the built-in object MyBase. For example, to refer to the existing Description() method of the Vehicle class, we could call MyBase.Description() from within the Description method of Car. This functionality allows you to provide additional functionality through overriding without having to redo all the original code as well.

In addition to marking code as Overridable, it is also possible to mark a method or property as MustOverride and a class as MustInherit. The MustOverride keyword indicates that any child of this class must provide its own version of this property or method, and the MustInherit keyword means that this class cannot be used on its own (you must base other classes off of it). It is important to note that if a class contains a method marked MustOverride, then the class itself must be marked as MustInherit.

Inheritance is a big topic, and we haven't covered it all; but with the information provided in here, you are ready to start designing some applications that take advantage of this object-oriented feature.

  • + 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