Introduction to Class Programming

Creating a class in Visual Basic is very simple: just select the Add Class Module command from the Project menu. A new code editor window appears on an empty listing. Visual Basic automatically add a class module named Class1, so the very first thing you should do is change the Class name in the Project Properties window in a more appropriate name. The first version of our class includes only a few properties. These properties are exposed as Public members of the class module itself.

• To create a class module 

1. Start Visual Basic. 
2. In the New Project dialog box, select Standard EXE, then click OK. 
3. On the Project menu, click Add Class Module. 
4. In the Add Class Module dialog box, select Class Module, then click Open. 
5. In the Properties window, set the Name property for the class module to Student.

• To create public property

1. In the Code window, type the following:

'In the declaration section of the Student class module
Public Student_ID As String
Public FirstName As String
Public LastName As String
Public MajorCode As String
Public YearLevel As String
Public BirthDate As Date

This is a very simple class, which consists of Public properties, so we are not distracted by OOP details (not yet), we will just examine the concept.  Once you've created a class, you can create an instance of that class then you can use the properties of that class. The following example creates an instance of the Student class, and sets and retrieves its properties: 

• To Use the Student Class 

1. Place a command button on Form1.
2. In the Click event for the command button, type the following: 

'Declare an object Student Dim objStudent As Student 'Create an instance of the class Set objStudent = New Student 'Use the object Student objStudent.StudentID = "12345" objStudent.FirstName = "Cathrina" objStudent.LastName = "Aniversario" objStudent.MajorCode = "C" objStudent.YearLevel = "Freshmen" objStudent.BirthDate = "Oct 10, 1980" MsgBox "Student ID : " & objStudent.StudentID & vbCrLf & _ "Student Name : " & objStudent.FirstName & " " & _ objStudent.LastName & vbCrLf & _ "Major Code : " & objStudent.MajorCode & vbCrLf & _ "Year : " & objStudent.YearLevel & vbCrLf & _ "BirthDate : " & objStudent.BirthDate Set objStudent = Nothing

3. On the Run menu in Visual Basic, click Start. 
4. When the program is running, click the Command1 button. 
5. Click OK to close the message box. 
6. On the Run menu in Visual Basic, click Stop. 

Unlike regular variable, which can be used as soon as they have been declared, an object variable must be explicitly assigned an object reference before you can invoke the object's properties and methods. When an object variable has not been assigned, it contains the special Nothing value, meaning it doesn't contain any valid reference to an actual object. For example try this code:

' Declare the variable
  Dim objStudent As Student

' Then comment out the next line
' Set objStudent = New Student 

' Raises an error 91 
' "Object variable or With block variable not set"
   MsgBox objStudent.FirstName

The code will give you an error, because we trying to use an object that doesn't exist.  This behavior is favorable because it doesn't make much sense to use a property of an object that doesn't exist. One way to avoid the error is to test its contents of an object variable using the Is Nothing

' Use the variable only if it contains a valid object reference
If Not (objStudent Is Nothing) Then MsgBox objStudent.FirstName

MsgBox objStudent.FirstName

But in other cases, you may want to create an object and then assign its properties. You might find it useful to declare an auto-instancing object variable using the As New clause:

Dim objStudent As New Student ' Auto-instancing variable

At runtime, when Visual Basic encounters a reference to an auto-instancing variable, it first determines whether it's pointing to an existing object and creates a brand new instance of the class if necessary. But auto-instancing variables have an advantage and disadvantage:

• It reduce the amount of code you need to write to be up and running with your classes. This can be useful if you are prototyping an application. 
• In some condition, you might declare a variable but never actually use it: which happens all the time with standard variables and with object variables too. In truth is, if you create an object with a Set command at the beginning of a procedure, you might be creating an object for no real purpose (thus taking both time and memory). On the other hand, if you delay the creation of an object until you actually need it, you could soon find yourself drowning in a sea of Set commands, each preceded by an Is Nothing test to avoid re-creating an object instanced previously. Auto-instancing variables are automatically created by Visual Basic only if and when they are referenced. This is probably the situation in which auto-instancing variables are most useful.
• Your object variable cannot be tested against the Nothing value. In fact, as soon as you use one in the Is Nothing test, Visual Basic insistently creates a new instance and the test always returns False
• It eliminate errors, which is sometimes this is specifically what you don't need especially during the development stage, because during this state, you want to see all the errors because this might be the symptoms of other serious deficiency in your code logic.
• Auto-instancing variables make the debugging step a little more difficult to understand because you can never be sure when and why an object was created.
• You can't declare an auto-instancing variable of a generic type, such as Object, or Form because Visual Basic must know in advance which kind of object should be created when it references that variable for the first time.
• Finally, each time Visual Basic references an auto-instancing variable, it incurs a small performance hit each time Visual Basic reference an auto-instancing, because Visual Basic has to check whether it's Nothing.

In short, auto-instancing variables are not the best choice for creating the object and you should stay away from it.

Properties Of a Class

Now is the time to make our class to more robust class. A robust class is one that actively protects its internal data from tampering. So how can a class protect itself from invalid assignments, such as an empty string for its FirstName or LastName properties. To accomplish this purpose, you must change the internal implementation of the class module, because in its present form you have no means of trapping the assignment operation. Simply change all the Public member into Private members and encapsulate them in pairs of Property procedures.

• To change our Student class 

1. Double click the class Student.cls in the Project Explorer 
2. In the Student Class Module, change all word Public to Private and add a prefix m_ in front of all private variables, as shown below: 

'In the declaration section of the Student class module Private m_Student_ID As String Private m_FirstName As String Private m_LastName As String Private m_YearLevel As String Private m_BirthDate As Date

NOTE

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You can also use Replace Dialog box. To do this, press Ctrl-H, the Replace Dialog box appears. On the Find What combo box, type Public. Next on the Replace With combo box, type Private, then click Replace All button. 

Appending the prefix m_ is just a personal style, this way it keeps my property name and private member variable synchronize and it is commonly used in programming. Feel free to use it or to create your own style.

3. In the Student Class Module, type the following code: 

'In the declaration section of the Student class module Private m_Student_ID As String Private m_FirstName As String Private m_LastName As String Private m_MajorCode As String Private m_YearLevel As String Private m_BirthDate As Date Property Get MajorCode() As String MajorCode = m_MajorCode End Property Property Let MajorCode(ByVal strNewValue As String) ' Raise an error if an invalid assignment is attempted. If Len(strNewValue) = 0 Or Len(strNewValue) > 1 Then Err.Raise 5 m_MajorCode = strNewValue End Property Property Get FirstName() As String FirstName = m_FirstName End Property Property Let FirstName(ByVal strNewValue As String) ' Raise an error if an invalid assignment is attempted. If Len(strNewValue) = 0 Then Err.Raise 5 ' Invalid procedure argument ' Else store in the Private member variable. m_FirstName = strNewValue End Property Property Get LastName() As String LastName = m_LastName End Property Property Let LastName(ByVal strNewValue As String) ' Raise an error if an invalid assignment is attempted. If Len(strNewValue) = 0 Then Err.Raise 5 ' Invalid procedure argument ' Else store in the Private member variable. m_LastName = strNewValue End Property Property Get StudentID() As String StudentID = m_Student_ID End Property Property Let StudentID(ByVal strNewValue As String) ' Raise an error if an invalid assignment is attempted. If Len(strNewValue) = 0 Then Err.Raise 5 ' Invalid procedure argument ' Else store in the Private member variable. m_Student_ID = strNewValue End Property Property Get BirthDate() As Date BirthDate = m_BirthDate End Property Property Let BirthDate(ByVal datNewValue As Date) If datNewValue >= Now Then Err.Raise 1001, , "Future Date!" m_BirthDate = datNewValue End Property Property Get YearLevel() As String YearLevel = m_YearLevel End Property Property Let YearLevel(ByVal strNewValue As String) Dim varTemp As Variant Dim found As Boolean For Each varTemp In Array("Freshmen", "Sophomore", "Junior", "Senior") If InStr(1, strNewValue, varTemp, vbTextCompare) Then found = True Exit For End If Next If Not found Then Err.Raise 5 m_YearLevel = strNewValue End Property

NOTE

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Visual Basic can help you in typing Property Procedure by Add Procedure command from the Tools menu, which creates a templates for Property Get and Let procedures. But you should edit the result to a proper data type, because all properties created by this command is of type Variant.

4. On the Run menu in Visual Basic, click Start. 
5. When the program is running, click the Command1 button. 
6. Click OK to close the message box. 
7. On the Run menu in Visual Basic, click Stop. 

Everything works as before. What we have done, is make the class a bit more robust because it now refuses to assign invalid values to its properties. To see what I mean, just try to issue this command:

objStudent.FirstName = ""	'Raises an error 'Invalid Procedure call