Home > Articles > Programming > Windows Programming

  • Print
  • + Share This
This chapter is from the book

Understanding Properties

All objects have attributes used to specify and return the state of the object. These attributes are properties, and you've already used some of them in previous hours using the Properties window. Indeed, every object exposes a specific set of properties, but not every object exposes the same set of properties. To illustrate this point, I'll continue with the hypothetical Pet object. Suppose that you have an object, and the object is a dog. This Dog object has a certain properties that are common to all dogs. These properties include attributes such as the dog's name, the color of its hair, and even the number of legs it has. All dogs have these same properties; however, different dogs have different values for these properties. Figure 3.1 illustrates such a Dog object and its properties.

Figure 3.1Figure 3.1 Properties are the attributes that describe an object.

Getting and Setting Properties

You've already seen how to read and change properties using the Properties window. The Properties window is available only at design time, however, and is used only for manipulating the properties of forms and controls. Most getting and changing of properties you'll perform will be done with Visual Basic .NET code, not by using the Properties window. When referencing properties in code, you specify the name of the object first, followed by a period (.), and then the property name, as in the following syntax:

{ObjectName}.{Property}

If you had a Dog object named Bruno, for example, you would reference Bruno's hair color this way:

Bruno.HairColor

This line of code would return whatever value was contained in the HairColor property of the Dog object Bruno. To set a property to some value, you use an equal sign (=). For example, to change the Dog object Bruno's Weight property, you'd use a line of code such as the following:

Bruno.Weight = 90

When you reference a property on the left side of an equal sign, you're setting the value. When you reference a property on the right side of the equal sign, you're getting (reading) the value. In the early days of BASIC, you actually used the word Let when setting values. This made the code easier to read for novices, but it was unnecessarily tedious. Nevertheless, using Let makes the statement clearer, so I'll show the same code statement as before with the word Let:

Let Bruno.Weight = 90

It's easier to see here that referencing the property on the left side of the equal sign indicates that you're setting the property to some value. The keyword Let is no longer a valid way to make variable assignments. If you enter a code statement that uses Let, you won't receive an error, but the code editor (also known as the gremlins) will automatically remove the word Let from the statement for you.

The following line of code places the value of the Weight property of the Dog object called Bruno into a temporary variable. This statement retrieves the value of the Weight property because the Weight property is referenced on the right side of the equal sign.

sngWeightVariable = Bruno.Weight

Variables are discussed in detail in Hour 11, "Using Constants, Data Types, Variables, and Arrays." For now, think of a variable as a storage location. When the processor executes this statement, it retrieves the value in the Weight property of the Dog object Bruno and places it in the variable (storage location) titled sngWeightVariable. Assuming that Bruno's Weight is 90, as set in the previous example, the computer would process the code statement like this:

sngWeightVariable = 90

Just as in real life, some properties can be read but not changed. Suppose that you have a Sex property to designate the gender of a Dog object. It's impossible for you to change a dog from a male to a female or vice versa (at least I think it is). Because the Sex property can be retrieved but not changed, it's known as a read-only property. You'll often encounter properties that can be set in design view but become read-only when the program is running.

One example of a read-only property is the Height property of the combo box control. Although you can view the value of the Height property in the Properties window, you can't change the value—no matter how hard you try. If you attempt to change the Height property using Visual Basic .NET code, Visual Basic .NET simply changes the value back to the default—eerie.

NOTE

The best way to determine which properties of an object are read-only is to consult the online help for the object in question.

Working with an Object and Its Properties

Now that you know what properties are and how they can be viewed and changed, you're going to experiment with properties in a simple project. In Hour 1, you learned how to set the Height and Width properties of a form using the Properties window. Now, you're going to change the same properties using Visual Basic .NET code.

The project you're going to create consists of a form with some buttons on it. One button will enlarge the form when clicked, whereas the other will shrink the form. This is a very simple project, but it illustrates rather well how to change object properties in Visual Basic code.

  1. Start by creating a new Windows Application project (from the File menu, choose New, Project).

  2. Name the project Properties Example.

  3. Use the Properties window to change the name of the form to fclsDrinkMe. (Click the form to once select it and you'll be able to change its Name in the Properties window.)

  4. Next, change the Text property of the form to Grow and Shrink.

When the project first runs, the default form will have a Height and Width as specified in the Properties window. You're going to add buttons to the form that a user can click to enlarge or shrink the form at runtime.

Add a new button to the form by double-clicking the Button tool in the toolbox. Set the new button's properties as follows:

Property

Set To

Name btnEnlarge
Text Eat Me
Location 111,70

Now for the Shrink button. Again, double-click the Button tool in the toolbox to create a new button on the form. Set this new button's properties as follows:

Property

Set To

Name btnShrink
Text Drink Me
Location 111,120

Your form should now look like the one in shown in Figure 3.2.

Figure 3.2Figure 3.2 Each button is an object, as is the form the buttons sit on.

To complete the project, you need to add the small amount of Visual Basic .NET code necessary to modify the form's Height and Width properties when the user clicks a button. Access the code for the Enlarge button now by double-clicking the Eat Me button. Type the following statement exactly as you see it here. Do not press the Enter key or add a space after you've entered this text.

Me.Width

When you typed the period, or dot, as it's called, a small drop-down list like the one shown in Figure 3.3 appeared. Visual Basic .NET is smart enough to realize that Me represents the current form (more on this in a moment), and to aid you in writing code for the object, it gives you a drop-down list containing all the properties and methods of the form. This feature is called IntelliSense, and it is relatively new to Visual Basic. When an IntelliSense drop-down box appears, you can use the up and down arrow keys to navigate the list, and press Tab to select the highlighted list item. This prevents you from misspelling a member name thereby reducing compile errors. Now that Visual Basic .NET is fully object-oriented, you'll come to rely on IntelliSense drop-down lists in a big way; I think I'd rather dig ditches than program without them.

Figure 3.3Figure 3.3 IntelliSense drop-down lists (also called auto-completion drop-down lists) make coding dramatically easier.

Use the Backspace key to completely erase the code you just entered and enter the following code in its place (press Enter at the end of each line):

Me.Width = Me.Width + 20
Me.Height = Me.Height + 20

You're probably wondering what you have to do with the width and height of the form. Actually the word Me doesn't refer to a person, it refers to the object to which the code belongs (in this case, the form). Me is a reserved word; it's a word that you can't use to name objects or variables because Visual Basic has a specific meaning for it. When writing code within a form module, as you're doing here, you should always use the reserved word Me rather than using the name of the form. Me is much shorter than using the full name of the current form, and it makes the code more portable (you can copy and paste the code into another form module and not have to change the form name to make the code work). Also, should you change the name of the form at any time in the future, you won't have to change references to the old name.

NOTE

Me works only in object-based modules such as form modules; you can't use Me in a standard module, which you'll learn about in Hour 10, "Creating and Calling Code Procedures."

The code you've entered does nothing more than set the Width and Height properties of the form to whatever the current value of the Width and Height properties happens to be, plus 20 pixels.

Redisplay the form designer by selecting the tab named Form1.vb [Design]. Then double-click the Shrink (Drink Me) button to access its Click event and add the following code:

Me.Width = Me.Width - 20
Me.Height = Me.Height - 20

This code is very similar to the code in the Enlarge_Click event, except that it reduces the Width and Height properties of the form by 20 pixels. Your screen should now look like Figure 3.4.

TIP

As you create projects, it's a very good idea to save frequently. Save your project now by clicking the Save All button on the toolbar.

The last step you need to perform is to specify the current form as the Startup object. Do this by right-clicking the Properties Example item in the Solution Explorer, choosing Properties, and then selecting fclsDrinkMe from the Startup object drop-down list. Click OK to close the Property Pages form.

Figure 3.4Figure 3.4 The code you've entered should look exactly like this.

Once again, display the form designer by clicking the tab Form1.vb [Design]. Your Properties Example project is now ready to be run! Press F5 to put the project in Run mode (see Figure 3.5).

Figure 3.5Figure 3.5 What you see is what you get—the form you created should look just as you designed it.

Click the Eat Me button a few times and notice how the form gets bigger. Next, click the Drink Me button to make the form smaller. When you've clicked enough to satisfy your curiosity (or until you get bored), end the running program and return to Design mode by clicking the Stop Debugging button on the toolbar.

  • + 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