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