Home > Articles > Programming > Windows Programming

This chapter is from the book

This chapter is from the book

Defining Interfaces

A Visual Basic .NET interface is neither a Visual Basic .NET class nor a COM class. The Component Object Model (COM) is language-neutral. Although Visual Basic .NET interfaces are also language-neutral, they represent an evolutionary progression from COM and ActiveX. Visual Basic .NET interfaces are managed code and COM is considered unmanaged code, meaning there is not a 1:1 mapping between COM and .NET interfaces. For now, suffice it to say that COM interfaces are not .NET interfaces. COM interop allows COM objects and .NET interfaces to interoperate; the subject of interoperation is discussed in Chapter 14, ".NET Framework." In this section we will be looking at how to define and implement Visual Basic .NET interfaces and comparing .NET interfaces to .NET classes.

Interfaces Compared to Classes

Interfaces in Visual Basic .NET define an interface only. Although Visual Basic .NET interfaces provide no implementation, classes can provide both an interface and an implementation. Visual Basic .NET classes provide an interface through abstract methods and an implementation through nonabstract methods.

There are only a few basic differences between interface and class grammar, as follows:

  • Interface members are public and class members can use any access specifier.

  • Interfaces use interface block and classes are defined in a class block.

  • Interfaces are implemented in classes as members tagged with the implements clause; class members use no special suffix clause.

  • Interfaces support multiple interface inheritance; classes support single inheritance only, but a single class may implement multiple interfaces.

  • Classes can inherit from one class and implement one or more interfaces in the same class; interfaces provide no implementation.

To keep the differences between interfaces and classes distinct in your mind, think of a class as describing a thing and an interface as describing a capabilities-adapter for a thing. For example, television is a good concept captured by a class. A cigarette lighter adapter for a television is a reasonable interface. A better interface for a television would be the capabilities of turning the television on and off and changing channels. The controls physically on the outside of the television could implement the control interface, as well as a remote control device implementing the control interface. The television and the remote control are two disparate things, but they both implement the interface necessary to control a television: TurnOn, TurnOff, ChangeChannel, and perhaps a method for selecting a specific channel.

The subsections that follow demonstrate how to define and implement an interface by implementing a television class and a control interface, and implementing the controls on both the television class and a remote control class.

Defining an Interface

The example consists of a Television class, a RemoteControl class, and a Control interface. Both the Television and RemoteControl classes implement the Control interface. The RemoteControl is initialized with a reference to a TV instance, simulating the infrared communication between TV and remote. The visual displays simulating the TV and remote are implemented using the Windows Media Player and Button controls (shown in Figure 10.7).

Figure 10.7 The form representing the television and remote control.

The purpose of this section is to demonstrate interfaces in Visual Basic .NET, so we will emphasize the Control interface in this section.

Listing 10.8 The Control interface listing.

 1: Public Interface Control
 2:  Sub Power()
 3:  Sub VolumeUp()
 4:  Sub VolumeDown()
 5:  Sub ChannelUp()
 6:  Sub ChannelDown()
 7:  Sub SelectChannel(ByVal Value As String)
 8:  Property Channel() As String
 9:  Sub Display()
10:  Event PowerChange(ByVal IsOn As Boolean)
11: End Interface

The Control interface demonstrates that interfaces are definitions without implementations. Every class that implements the Control interface must implement all of the interface members. Notice that there are no access specifiers; every member of an interface is public.

Implementing an Interface

A remote control passes requests from a TV viewer to the television. For our purposes a reasonable implementation is to implement the control interface and send the user-request to the TV (simulating an infrared signal). Listing 10.9 demonstrates the RemoteControl class, which implements the Control interface.

Listing 10.9 The RemoteControl class implements the Control interface.

 1: Public Class RemoteControl
 2:  Implements Control
 3:  Private FTV As Television
 5:  Public Event PowerChange(ByVal IsOn As Boolean) _
 6:   Implements Control.PowerChange
 8:  Protected Overridable Sub OnPowerChange()
 9:   RaiseEvent PowerChange(FTV.IsOn)
10:  End Sub
12:  Public Sub New(ByVal TV As Television)
13:   MyBase.New()
14:   FTV = TV
15:  End Sub
17:  Public Sub ChannelDown() Implements Control.ChannelDown
18:   FTV.ChannelDown()
19:  End Sub
21:  Public Sub ChannelUp() Implements Control.ChannelUp
22:   FTV.ChannelUp()
23:  End Sub
25:  Public Sub VolumeDown() Implements Control.VolumeDown
26:   FTV.VolumeDown()
27:  End Sub
29:  Public Sub VolumeUp() Implements Control.VolumeUp
30:   FTV.VolumeUp()
31:  End Sub
33:  Public Sub SelectChannel(ByVal Value As String) _
34:   Implements Control.SelectChannel
35:   FTV.SelectChannel(Value)
36:  End Sub
38:  Public Property Channel() As String Implements Control.Channel
39:   Get
40:    Return FTV.Channel
41:   End Get
42:   Set(ByVal Value As String)
43:    FTV.Channel = Value
44:   End Set
45:  End Property
47:  Public Sub Display() Implements Control.Display
48:   FTV.Display()
49:  End Sub
51:  Public Sub Power() Implements Control.Power
52:   FTV.Power()
53:   OnPowerChange()
54:  End Sub
55: End Class

Line 2 indicates that RemoteControl is going to implement all of the members of the Control interface. Line 3 maintains a reference to the television set simulating infrared communication. (We could add an infrared transmitter and receiver if it suited our needs; we do not need to go that far to demonstrate interfaces, though.) If you compare the number of methods that have Implements clauses, you will see that there is a one-to- one relationship between interface members in the Control interface and methods with Implements clauses in the RemoteControl class. You must implement every member of an interface.


Notice that methods that implement an interface are public in Visual Basic .NET. You can call interface methods from consumers in Visual Basic .NET. For example, you could create an instance of the RemoteControl class and call the interface methods directly or access the interface methods via a reference to the interface type.

In VB6, implemented interface methods were private and consumers could not call them directly.

Notice that we can define and implement interfaces with methods, properties, and events. The Control interface contains several methods, the Channel property, and a PowerChange event.

The most significant difference between interface definitions and implementations—refer to Listings 10.8 and 10.9—is that interfaces do not contain implementations of members.

Interface Inheritance

Interfaces support inheritance too. You can define an interface and define a second interface that inherits from the original interface. By adding new interface members to the new interface, you can extend the definition of an interface or inherit from multiple interfaces.

Any class that implements an interface must implement all of the members of the interface. Refer back to Listing 10.3, which demonstrated the syntax for multiple interface inheritance.

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.


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.


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.


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.


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


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


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.


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.


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