Objects Fundamentals

Objects

Introduction

An object is anything that can be described. In the real world, examples of objects are the moon, a ball, a hand, a book, a head, a song, a box. As different as objects are, they follow some basic rules used to describe them. A characteristic of an object is a word or a group of words used to describe the object. Some characteristics are applied to all objects. For example, every object must be identified with a word or a group of words referred to as its Name:

Object
Name	Person	Table	Ball	Insect	Map

Some characteristics apply to a group of objects but don't apply to another group. For example, a characteristic called width can be used to describe a car, a medical pill, a piece of paper, or a computer monitor:

Common Characteristics
Name	Digital Camera	Basket	Car	Tennis Ball
External Color	Black	Blue	Grey	Yellow
Unit Price	899.95	24.55	19995.00	6.25

Some other characteristics can be applied to one object or one type of object that is mostly unique.

When creating a database, you will also use objects but these are referred to as Windows controls or simply controls. In a typical application, you will choose the objects, that is, the controls that you judge necessary and you will make them part of your application. Here is an example of a form with various controls:

As we move, we will indicate how to choose controls and populate a form with them.

1. Start Microsoft Access and, from the resources that accompany these lessons, open the Exercise1 database
2. To display an existing object, under the Objects bar of the Database window, click Forms
3. In the right frame of the window, double-click Sample Form
    
   

In the programming world, a characteristic of an object is referred to as a property of that object. For example, as mentioned above, every object must have a name. The name is used to identify the object. In the same way, the other characteristics that we reviewed above are in fact the properties of that object.

Because every object has properties, they can be created as a list. Consider the second table we saw above:

Common Characteristics
Name	Digital Camera	Basket	Car	Tennis Ball
External Color	Black	Blue	Grey	Yellow
Unit Price	899.95	24.55	19995.00	6.25

The properties of each object are: its name, its external color, and its unit price. This can be illustrated as:

To represent an object, that is, to describe it, you can give a value to each property. For example, the properties of the digital camera from the above table are its Name, its External Color, and its Unit Price. The values of the properties of that camera are: Name: Digital Camera, External Color: Black, Unit Price: 899.95. This can be illustrated as:

From this illustration, it is important to make a distinction between a property and its value: a property is a word or a group of words used to define what constitutes an object. A value is the word or a group of words used to formally describe an object. In the programming world, the name of a property is always of one word only, as the Name property in the above table. If a name is made of more than one word, then they must be combined into one. In the same way, the value of a property is made of only one word. Also, if the name is a combination of words, they must be concatenated (added) to produce one word. Based on this, the properties and their values from the above table would be:

Just as done in the real world, Microsoft Access also relies on objects to represent a database. One of the most regularly used objects of a database is called a table. Another regularly used object of an application is called a form. There are many other objects as we will find out in future lessons.

Each object has properties. To effectively use an object, you should be familiar with its properties or at least how to use them to describe an object.

During the design of an object, you will have a window that represents its properties. The window that displays the list of properties is different depending on the object and depending on whether you are working in Microsoft Access or in Microsoft Visual Basic. For a form, which is probably the most regularly used object of an application, in these lessons, the window that presents its properties will be called the Properties window. It appears as a resizable horizontal window with 5 tabs labeled Format, Data, Event, Other, and All:

As mentioned in the previous lesson, in Microsoft Visual Basic, the Properties window usually appears in the lower left section of the screen and appears with 2 tabs labeled Alphabetic and Categorized:

To visually configure a property, you must first locate it in the Properties window. If you are working in Microsoft Access, the properties are categorized in three tabs: Format, Data, and Other. All of these properties are also represented in the All tab. Each property appears with its name as in the real world: in different words. Examples are Caption, Default View, or Min Max Buttons. After locating the property, to see or change its value, you use the box on its right. This means that a property is made of two sections: a property name and a property value. This can be illustrated as follows:

If you are working in Microsoft Visual Basic, you can use the same approach to change a property using the Properties window. This time, the names of properties appear in one word and they are in their official format.

To programmatically change a property, in your code section, type the name of its object, followed by a period operator ".", followed by the official name of the property, followed by the assignment operator, and followed by the desired value. This means that you must know the name of the object whose property you want to change. You must know the name of the property you want to change, and you must know the possible values that the property can receive. Here is an example of code that hides a rectangular box, named boxRectangle, when the user clicks a button:

Private Sub cmdHide_Click()
    boxRectangle.Visible = False
End Sub

Fortunately, when writing code, the Code Editor is equipped to assist you with the names of available properties. When you type the name of a object followed by the period operator, the available properties would appear in a list:

We mentioned earlier that the names of objects are usually in one word. In reality, Microsoft Access is very flexible and allows you to use more than one word to name an object (but the properties names are always in one word). If you have an object that is made of more than one word, when referring to it in an expression, whether in the Properties window or with your code, you must include it between the opening square bracket "[" and the closing square bracket "]". For example, suppose that you have a box named Rectangular Box instead of boxRectangle. The above code would be written as:

Private Sub cmdHide_Click()
    [Rectangular Box].Visible = False
End Sub

There is no penalty if you always include the name of an object in square brackets when referring to it whether in the Properties window or with code, even if the name is made of one word. Here is an example:

Private Sub cmdHide_Click()
    [boxRectangle].Visible = False
End Sub

This would produce the same result as above.

1. To change the view of the form, while the form is activated, on the main menu, click View -> Design View
2. Notice that a window appears with a caption that displays Form. If you don't see it, on the main menu of Microsoft Access, click View -> Properties
    
   
3. This is the Properties window in Microsoft Access. Notice that it is divided in five property pages
4. To open Microsoft Visual Basic, on the Standard toolbar of Microsoft Access, click the Code button
5. Notice that the Properties window displays the properties of the form as it is selected in the Project window
6. In the Object combo box of the Code Editor, select Form
7. Type NavigationButtons = False
    
   
8. To return to Microsoft Access, on the toolbar, click the View Microsoft Access button
9. On the Standard toolbar of Microsoft Access, click the View button
    
   
10. After viewing the result, on the Form View toolbar of Microsoft Access, click the View button

Sometimes, you will need to access only one property of an object. In some other cases, you will need to change various properties to perform a specific task. To do this for each property, as we saw above, you can type the name of the object, followed by the period operator, followed by the name of the property, press Enter, and do the same on the next line. Here is an example:

Private Sub cmdManipulate_Click()
    boxEnvelop.BackStyle     = 1
    boxEnvelop.BackColor     = 979478
    boxEnvelop.SpecialEffect = 1
    boxEnvelop.BorderColor   = 234657
    boxEnvelop.BorderWidth   = 2
End Sub

As an alternative, instead of typing the name of the control over and over again, you can use the With operator whose formula is:

With ObjectName
	Statements
End With

On the right side of the With keyword, type the name of the control whose properties you want to access. Under the With ObjectName line, type your statements and expressions as you wish but start each property of the ObjectName with the period operator ".". At the end of the With statement, type End With. Based on this, the above code would have been written:

Private Sub cmdManipulate_Click()
    With boxEnvelop
    	.BackStyle     = 1
    	.BackColor     = 979478
    	.SpecialEffect = 1
    	.BorderColor   = 234657
    	.BorderWidth   = 2
    End With
End Sub

Because properties are meant to accomplish various goals, they are also configured differently. In previous sections, we saw that the name of a property was represented on the left column of a tab in the Properties window and the value of the property was on the right side:

There are various kinds of fields you will use to set the properties. To know what particular kind a field is, you can click its name. To set or change a property, you use the box on the right side of the property’s name.

There are fields that expect you to type a value. Most of these fields have default text. To visually change the value of the property, click the name of the property, type the desired value, and press Enter. While some properties, such as the Text or Caption, would allow anything, some other fields expect a specific type of text, such as a numeric value. To programmatically change the value of the property, use the approach we described above and assign it the desired value. At design time, that is, in the Properties window, the text you provide will appear "as is" and cannot change. If you want the text to change in response to something, you must write code.

1. While the form is displaying in Microsoft Access, if the Toolbox is not displaying, on the main menu, click View -> Toolbox.
   In the Toolbox, click Text Box and click (somewhere in) the top section of the form
2. While the new text box is still selected, in the Properties window, click the Other tab and notice that there is a default text in the Name field.
   Click Name, type txtToday and press Enter
3. In the Toolbox, click the Command Button and click somewhere on the right side of the text box that was added.
   If a dialog box comes up (it should), click Cancel
4. While the text box is still selected, in the All tab of the Properties window, click Name and type cmdToday
5. Click Caption and type Today
6. In the Form View toolbar, click the Code button
7. Click the arrow of the Object combo box and select cmdToday
8. Type [txtToday] = Date
    

   Private Sub cmdToday_Click() [txtToday] = Date End Sub

9. To return to Microsoft Access, on the Standard toolbar, click View Microsoft Access
10. Once in Microsoft Access, click the View button on the Form View toolbar
11. When the form displays, click the button and notice that the text box displays the current date
12. To switch back to Form view, click the View button on the Form View toolbar

Some fields expect a numeric value. In this case, you can click the name of the field and type the desired value. If you type an invalid value, you would receive a message box notifying you of the error:

When this happens, click OK and type a valid value. If the value is supposed to be an integer, make sure you don't type it as a decimal number.

1. On the form, click the Detail bar
2. In the Properties window, click the Format tab and click Height
3. Type 1.85 and pres Enter

By default, these fields have nothing in their properties. Most of these properties are depending on other characteristics of an object. To visually set the property on such a field, you can type in it or select from a list. To programmatically set the property, use the approach we reviewed earlier: the name of the property, the assignment operator, and the desired value.

1. Click the square button in the top-left section of the form under the title bar
2. In the Properties window, click the Format tab and click Caption
3. Type Department of Records and Statistics and press Enter
4. Switch the form to Form View to see the result
5. Switch it back to Design View

Some fields can have only a Yes or a No value. To change their setting, you can either select from the combo box or double-click the property to toggle to the other value.

1. While the properties of the form are still displaying in the Properties window, click the Format tab and double-click Record Selectors to change its value from Yes to No
2. Press the down arrow key a few times to reach the Auto Center field
3. Type y and notice that it changes to yes
     
   
4. Save the form

Some fields would require a value or item that needs to be set through an intermediary action. Such fields display an ellipsis button . When you click the button, a dialog box would come up and you can set the value for the field.

1. Right-click the following picture and click Copy
    
   
2. Open Microsoft Paint (Start -> (All) Programs -> Accessories -> Paint) and paste the picture in in. Close Microsoft Paint. When asked whether you want to save it, click Yes and save the picture as Background in your My Documents folder. Return to Microsoft Access
3. Click the square button in the top-left section of the form under the title bar
4. In the Properties window, click the Format tab and click Picture
5. Click the ellipsis button of the Picture field and, in the Insert Picture dialog box, locate the folder where you saved the above picture
6. Select the picture and click OK
7. To preview the form, switch it to Form Views window, click Icon and click its ellipsis button of the Pi
8. Locate the Drive:\Program Files\Microsoft Visual Studio .NET 2003\Common7\Graphics\icons\arrows folder and display it in the Look In combo box
    
   
9. After viewing it, switch the form back to Design View

To change the value of some of the fields, you would use their combo box to display the available values. After clicking the arrow, a list would display. Here is an example:

There are various types of list-based fields. Some of them display just two items. To change their value, you can just double-click the field. Some other fields have more than two values. To change them, you can click their arrow and select from the list. You can also double-click a few times until the desired value is selected.

1. On the form, click the Unbound text box. This would allow you to locate its accompanying label
2. On the left side of the Unbound text box, click the label (it appears with eight sizing handles)
3. In the Properties window, click the Format tab and click Font Weight
4. Click the arrow of Font Weight and select Semi-Bold
5. Click Fore Color and click its ellipsis button
6. In the Color dialog box, click the yellow button (2nd column - 2nd row) and click OK
7. In the Properties window, if necessary, scroll up and click Caption
8. Type Today: and press Enter
9. Switch the form to Form View to preview it
10. Save and close the form

As opposed to being described, an object can perform actions. For example, a Person object can sing. A Car object can move. An Insect object can crawl. A Basket object can hold some clothes. Here are examples of actions:

Objects	Person	Car	Dog	Insect
Actions	Move	Move	Move	Move
	Talk Aloud	Protect (from rain)	Protect (from intruders)	Crawl
	Sing	Carry	Sit Down	Smell Food
	Eat		Eat	Eat
	Walk Slowly		Bark	Walk

Notice that some objects can perform similar types of actions. For example all objects of this table can move. Some objects can perform actions that some others can't. For example, from the objects in the above table, a person and a dog can eat.

In the programming world, an action that an object can perform is called a method. Like a property, a method must have a name. On the above table, notice that the name of a method usually (if not always) resembles a verb. Examples are Bark, Eat, or Drink. To make a distinction between a property and a method, in this ebook, we will always follow the name of a method with empty parentheses. Examples are Move() or Bark().

Like a property, the name of a method is always in one word. Examples are Walk() or Crawl(). If the name is a combination of words, each part starts in uppercase. Examples would be TalkAloud(), SmellFood(), EatGrass(), or ProtectFromRain().

Because a method is performed, it cannot be represented in a window such as the Properties window. Eventually, we will know how to identify and use the methods of an object. Using a method is referred to as calling it. When a method can produce a result, it can be assigned to a property.

To call a method, if it produces a result that can be applied to a property, if you are working in Microsoft Access, in the Properties window, locate the property that will use the result of the method. In the property value section, type the assignment operator "=", followed by the name of the object that owns the method, followed by a period operator, followed by the name of the method and followed by parentheses. In future lessons, we will see that there can be other issues involved with calling a method.

To programmatically call a method, type the name of the object that owns the method, followed by a period operator, and followed by the name of the method.

1. Once again, in the Toolbox, click Text Box and click the middle section of the form
2. While the middle text box is still selected, in the Properties window, click the Other tab
3. Click Name and type Company Name and press Enter
4. To preview the form, on the Standard toolbar, click the View button and notice that the caret is blinking in the top text box, indicating that it has focus
5. To return to design view, on the Standard toolbar, click the View button
6. To return to Visual Basic, on the task bar, click Microsoft Visual Basic
7. In the Code Editor, click the right side of the first line of code and press Enter
8. Type [Company Name].SetFocus
    

   Private Sub Form_Load() NavigationButtons = False [Company Name].SetFocus End Sub

9. To return to Microsoft Access, on the task bar, click Main Form : Form
10. On the Standard toolbar of Microsoft Access, click the View button
11. Notice that, this time, it is the middle text box that has the blinking caret because it has focus.
    After viewing the result, on the Standard toolbar of Microsoft Access, click the View button

A typical application is made of various objects that a person uses to interact with the computer. To make this interaction possible without confusion, each object creates its own messages and sends them to the operating system. The messages are as varied as possible, so are the objects of an application. To reduce any type of confusion, each message must carry three to four pieces of information: the name of the object that composed the message, the type of message that must be dealt with, additional information provided by the object that sent the message.

Because the operating system cannot decide what type of action a particular object needs to perform at a particular time, each object is responsible to create its own messages. The first piece of information that an object must provide is its name. The reason is that there can be so many objects that are part of an application and there can be many applications opened at the same time.

The second piece of information that a message must carry is its type. There are many types of messages that a control can compose. Many controls can also send the same types of messages. The operating system is already aware of the various types of messages that are available and, most of the time, it knows the types of messages that a particular object can control, but the operating system cannot decide what type of message a control wants to compose at one particular moment. The types of messages are mostly known with particular names. In the operating system, the names of messages start with WM_. If you log to the MSDN web site and do a search on words that start with WM_, you would find out that there are many messages available but you will not need to memorize their names or to be aware of all of them. In fact, there are many messages you will hardly use.

After a control has composed a message, it must send it. The action of composing and sending a message is called an event. The action of actually sending a message is referred to as firing an event. To make it easy to identify an event, each event has a simplified name. When an object fires an event, the name of the event is appended to the name of the object. To distinguish the name of the object and the name of the event, there is an underscore between them. Examples are MainForm_Load or Customers_Dirty. As you will find out, the names of some events are made of more than one word. Examples are AfterUpdate or BeforeInsert. In this case, the name of the object and the name of the event are still added with an underscore between them. Examples are MainForm_AfterUpdate or Customers_BeforeInsert.

After composing a message and firing its event, when the operating system receives it, in order to perform the necessary action, in some cases, it (the operating system) may need additional information that the object must provide. The additional information is also referred to as parameter.

The additional information that an object must provide is typed in the parentheses of the event. Normally, the operating system is aware of the type of additional information that the object must provide. If the object doesn't provide this information, the operating system would use some default value(s). Because the type of additional information of an event is already decided, when you generate an event, Microsoft Visual Basic includes this information in the parentheses of the event. If the message doesn't require additional information, its parentheses are left empty. If it may need parameters, they are automatically included in its parentheses.

Because the structure of messages can be difficult to cope with, Microsoft Visual Basic has a simplified mechanism to help you code an event. Since an event is treated as a private matter, its code starts with the Private keyword. An event is treated as an action, like the methods we reviewed earlier. For this reason, it is created as a sub procedure and it must use the Sub keyword. After the Private Sub expression, follow the combination of the name of the object that is firing the event, the underscore, and the name of the event, and its parentheses with optional parameters depending on the event.

There are various ways you can initiate an event on an object of your database. If you are working in Microsoft Access and if you open a form or a report, you should access its Properties window in Design View. If the form or report "carries" the control whose event you want to program, you should also first open the form or report in Design View.

• First Technique
  

  1. If you want to write code that relates to the whole form or event, double-click its selection button which is at the intersection of the rulers .
     If you want to write code for a control, on the form or report, double-click the control to display its Properties window.

  2. Once the Properties window is displaying, you can click the Event property page and inspect the list of events:
      
     
     

  3. After locating the event you want, double-click it. An [Event Procedure] string will be added to the field:
      
     
      
     Once the [Event procedure] is displaying, click the ellipsis button to launch Microsoft Visual Basic. The keyboard caret would be positioned in the event and wait for you.

• Second Technique

  1. In the Properties window, click the name of the event in the Event property page to reveal its combo box

  2. Click the arrow of the combo box and click [Event procedure]

  3. Once [Event Procedure] is displaying for an event, click the ellipsis button of the event . This will launch Microsoft Visual Basic and positions the caret in the event's body, waiting for your coding instructions.

• Third Technique

  1. Right-click the object whose event you want to write code for

  2. Click Build Event...
      
     

  3. On the Choose Builder dialog box, click Code Builder and click OK. This will launch Microsoft Visual Basic with the default event of the form, report, or control

• Fourth Technique

  1. First open or display the object in Microsoft Access

  2. To launch Microsoft Visual Basic, click the Code button

  3. In the Object combo box, select the object to launch its default event

  4. In the Procedure combo box, select the event you want.

Sometimes you or Microsoft Visual Basic will have inserted an event that you didn't want or that you don't want to program. If this happens, simply ignore the event: you do not have to delete it because if an event has been initiated but no code was written for it, the application will ignore it and use a default behavior.

1. To return to Microsoft Visual Basic, on the task bar, click Microsoft Visual Basic - Exercise1
2. Notice that the Object combo box is displaying the word Form.
   Click the arrow of the Procedure combo box to display the events of the form:
    
   
3. Click Open.
    
   
4. Click the arrow of the Procedure combo box again but this time, select ApplyFilter.
   Notice that the Load takes no parameter, the Open event takes one parameter, and the ApplyFilter takes 2 parameters
    
   
5. Click the arrow of the Object combo box and notice that Company_Name is in one word
    
   
6. Click Company_Name
7. Click the arrow of the Procedure combo box and select Change to generate that event