VBACreating a Custom Class

Remarks

This article will show how to create a complete custom class in VBA. It uses the example of a DateRange object, because a starting and ending date are often passed together to functions.

Adding a Property to a Class

A Property procedure is a series of statement that retrieves or modifies a custom property on a module.

There are three types of property accessors:

  1. A Get procedure that returns the value of a property.
  2. A Let procedure that assigns a (non-Object) value to an object.
  3. A Set procedure that assigns an Object reference.

Property accessors are often defined in pairs, using both a Get and Let/Set for each property. A property with only a Get procedure would be read-only, while a property with only a Let/Set procedure would be write-only.

In the following example, four property accessors are defined for the DateRange class:

  1. StartDate (read/write). Date value representing the earlier date in a range. Each procedure uses the value of the module variable, mStartDate.
  2. EndDate (read/write). Date value representing the later date in a range. Each procedure uses the value of the module variable, mEndDate.
  3. DaysBetween (read-only). Calculated Integer value representing the number of days between the two dates. Because there is only a Get procedure, this property cannot be modified directly.
  4. RangeToCopy (write-only). A Set procedure used to copy the values of an existing DateRange object.
Private mStartDate As Date                ' Module variable to hold the starting date
Private mEndDate As Date                  ' Module variable to hold the ending date
  
' Return the current value of the starting date
Public Property Get StartDate() As Date
    StartDate = mStartDate
End Property

' Set the starting date value. Note that two methods have the name StartDate
Public Property Let StartDate(ByVal NewValue As Date)
    mStartDate = NewValue
End Property
  
' Same thing, but for the ending date
Public Property Get EndDate() As Date
    EndDate = mEndDate
End Property
  
Public Property Let EndDate(ByVal NewValue As Date)
    mEndDate = NewValue
End Property

' Read-only property that returns the number of days between the two dates
Public Property Get DaysBetween() As Integer
    DaysBetween = DateDiff("d", mStartDate, mEndDate)
End Function

' Write-only property that passes an object reference of a range to clone
Public Property Set RangeToCopy(ByRef ExistingRange As DateRange)

Me.StartDate = ExistingRange.StartDate
Me.EndDate = ExistingRange.EndDate

End Property

Adding Functionality to a Class

Any public Sub, Function, or Property inside a class module can be called by preceding the call with an object reference:

Object.Procedure

In a DateRange class, a Sub could be used to add a number of days to the end date:

Public Sub AddDays(ByVal NoDays As Integer)
    mEndDate = mEndDate + NoDays
End Sub

A Function could return the last day of the next month-end (note that GetFirstDayOfMonth would not be visible outside the class because it is private):

Public Function GetNextMonthEndDate() As Date
    GetNextMonthEndDate = DateAdd("m", 1, GetFirstDayOfMonth())
End Function

Private Function GetFirstDayOfMonth() As Date
    GetFirstDayOfMonth = DateAdd("d", -DatePart("d", mEndDate), mEndDate)
End Function

Procedures can accept arguments of any type, including references to objects of the class being defined.

The following example tests whether the current DateRange object has a starting date and ending date that includes the starting and ending date of another DateRange object.

Public Function ContainsRange(ByRef TheRange As DateRange) As Boolean
    ContainsRange = TheRange.StartDate >= Me.StartDate And TheRange.EndDate <= Me.EndDate
End Function

Note the use of the Me notation as a way to access the value of the object running the code.

Class module scope, instancing and re-use

By default, a new class module is a Private class, so it is only available for instantiation and use within the VBProject in which it is defined. You can declare, instantiate and use the class anywhere in the same project:

'Class List has Instancing set to Private
'In any other module in the SAME project, you can use:

Dim items As List
Set items = New List

But often you'll write classes that you'd like to use in other projects without copying the module between projects. If you define a class called List in ProjectA, and want to use that class in ProjectB, then you'll need to perform 4 actions:

  1. Change the instancing property of the List class in ProjectA in the Properties window, from Private to PublicNotCreatable

  2. Create a public "factory" function in ProjectA that creates and returns an instance of a List class. Typically the factory function would include arguments for the initialization of the class instance. The factory function is required because the class can be used by ProjectB but ProjectB cannot directly create an instance of ProjectA's class.

     Public Function CreateList(ParamArray values() As Variant) As List
         Dim tempList As List
         Dim itemCounter As Long
         Set tempList = New List
         For itemCounter = LBound(values) to UBound(values) 
             tempList.Add values(itemCounter)
         Next itemCounter
         Set CreateList = tempList
     End Function
    
  3. In ProjectB add a reference to ProjectA using the Tools..References... menu.

  4. In ProjectB, declare a variable and assign it an instance of List using the factory function from ProjectA

     Dim items As ProjectA.List
     Set items = ProjectA.CreateList("foo","bar")
    
     'Use the items list methods and properties
     items.Add "fizz"
     Debug.Print items.ToString()
     'Destroy the items object
     Set items = Nothing