Variant Properties
You can add flexibility to your class by including a Variant member.
Assuming that you want to implement a ProvincialAddress property, but you
want to keep it more flexible and capable of storing either a Address
object or a string. Now let us add ProvincialAddress property
to our Student class as shown below:
Private m_ProvincialAddress As Variant
Property Get ProvincialAddress() As Variant
If IsObject(m_ProvincialAddress) Then
Set ProvincialAddress = m_CurrentAddress ' Return a Address object.
Else
ProvincialAddress = m_ProvincialAddress ' Return a string.
End If
End Property
Property Let ProvincialAddress(ByVal strNewProvincialAddress As Variant)
m_ProvincialAddress = strNewProvincialAddress
End Property
Property Set ProvincialAddress(ByVal strNewProvincialAddress As Variant)
Set m_ProvincialAddress = strNewProvincialAddress
End Property
' Revised StudentInfo method
Function StudentInfo(Optional ByVal IncludeAddressInfo As Boolean = True, _
Optional ByVal IncludeProvincialAddressInfo As Boolean = False) As String
' Returns the Student information
Dim info As String
info = "Student # : " & StudentID & vbCrLf & _
"Name : " & FullName & vbCrLf & _
"Age : " & Age & vbCrLf & _
"Gender : " & GenderDescription & vbCrLf & _
"Major Code : " & MajorCode & vbCrLf & _
"Major Description: " & MajorCodeDescription & vbCrLf & _
"Year Level : " & YearLevelDescription
If IncludeAddressInfo Then info = info & vbCrLf & "Address : " & StudentAddressInfo()
If IncludeProvincialAddressInfo Then info = info & vbCrLf & "Provincial Add : " & _
StudentProvincialAddInfo()
StudentInfo = info
End Function
' Newly added method for Student class
Function StudentProvincialAddInfo() As String
' Return Student Provincial address
If IsObject(m_ProvincialAddress) Then
' invoke Address CompleteAddress method
StudentProvincialAddInfo = m_ProvincialAddress.CompleteAddress
Else
' simply return a string
StudentProvincialAddInfo = m_ProvincialAddress
End If
End Function
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But things are a bit more complex if the property can receive either a
regular value or an object value. While this sort of flexibility adds a lot of power to your class, it also
reduces its robustness because nothing keeps a programmer from adding a
nonstring value or an object of a class other than Address:
'In the client form
With Student
.FullName = "Dante Salvador"
Set .Address = 12345 ' an Integer value
.StudentID = "102472"
'etc
End With
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Because ProvincialAddress property is declared as Variant type,
meaning you can assign any value, including numeric type. To have more
control of what is actually assigned to this property, you need to arbitrate all
accesses to it through Property procedures:
' In Student class module
' Revised property procedures
Property Let ProvincialAddress(ByVal strNewProvincialAddress As Variant)
' Check if it is a string value.If VarType(strNewProvincialAddress) <> vbString Then Err.Raise 5
m_ProvincialAddress = strNewProvincialAddress
End Property
Property Set ProvincialAddress(ByVal strNewProvincialAddress As Variant)
' Check if it is a Address object.
If TypeName(strNewProvincialAddress) <> "Address" Then Err.Raise 5
Set m_ProvincialAddress = strNewProvincialAddress
End Property
'In the client form
With Student
.FullName = "Dante Salvador"
Set .Address = 12345 ' this raises an error
.StudentID = "102472"
'etc
End With
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Another technique that you can use that give slightly improve run-time
performances and you save some code is to declare the type of the object you're expecting
right in the parameter list of the Property Set procedure:
Property Set ProvincialAddress(ByVal strNewProvincialAddress As Address)
' Check if it is a Address object.
If TypeName(strNewProvincialAddress) <> "Address" Then Err.Raise 5
Set m_ProvincialAddress = strNewProvincialAddress
End Property
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But you can't use
it when if your class accept two or more objects of different types. One
solution is use As Object parameter:
Property Set ProvincialAddress(ByVal strNewProvincialAddress As Object)
If TypeName(strNewProvincialAddress) <> "Address" And _
TypeName(strNewProvincialAddress) <> "OtherAddressType" Then Err.Raise 5
Set m_CurrentAddress = newValue
End Property
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Object Keyword
The TypeName function
The TypeName function returns the name of an object's class in
the form of a string. This means that you can find the type of an object
in a more concise form
In many situations, testing an object's type using the TypeName
function is preferable to using the TypeOf...Is statement because
it doesn't require that the object class be present in the current
application or in the References dialog box. For information about
TypeOf...Is function, consult your Visual Basic documentation.
The VarType function
The VarType function returns the type name of an object's
class in the form of a string. Variant variables can also host special
values that don't correspond to any data values described so far. The Emptyvalue is the state of a Variant variable when nothing has been
assigned to it yet. You can test this special value using the IsEmpty
function, or you can test the VarType function for the value
0-vbEmpty.
The Null value is useful in database programming to mark fields that
don't contain a value. You can explicitly assign the Null value to a
Variant using the Nullconstant, test for a Null value using the IsNull
function, or compare the return value of the VarType function
with the value 1-vbNull. For more information about this function,
consult your Visual Basic documentation.
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