Properties

These are the quantifiable characteristics of an object.
You can find out whether an exception will be generated by looking in the object model documentation.
Property values can be Numerical, String or Boolean and they may be specific or apply to several objects.
They can be obtained or set by assignment statements.
You can easily use the NOT operator to toggle properties on or off. (eg ActiveWindow.DisplayAlerts = NOT ActiveWindow.DisplayAlerts).

Declarations

Also known as property procedures.

Property Get - used to retrieve a property of a class. Get the value of scalar of object property.
Lets you return a property value or property reference
Take action before retrieving
Return a calculated value

Property Let - used to assign the value to a property of a class. Set the value of scalar property.
Lets you define a new property value
Take action after setting
Perform data validation

Property Set - used to assign an object to a property of a class. Set a reference for object property.
Lets you define a new property reference

Property - Collection

Private field_colValue As Collection 

Property Get Property_Name() As Collection 
    Set Property_Name = field_colValue 
End Property 

' You would use Set because Collection is a reference data type
Property Set Property_Name(colData As Collection) 
    Set field_colValue = colData 
End Property 

Property - Object

Private field_oValue As Object 

Property Get Property_Name() As Object 
    Set Property_Name = field_oValue 
End Property 

' You would use Set because Object is a reference data type
Property Set Property_Name(oData As Object) 
    Set field_oValue = oData 
End Property 

Property - Date

Private field_dtValue As Date 

Property Get Property_Name() As Date 
    Property_Name = field_dtValue 
End Property 

' You would use Let because Date is a value data type
Property Let Property_Name(dtData As Date) 
    field_dtValue = dtData 
End Property 

Property - String

Private field_sValue As String 

Property Get PropertyName() As String 
    PropertyName = field_sValue 
End Property 

' You would use Let because String is a value data type
Property Let PropertyName(sData As String) 
    field_sValue = sData 
End Property 

Property - Variant

Private field_vValue As Variant 

Property Get Property_Name() As Variant 
    If IsObject(field_vValue) Then 
        Set Property_Name = field_vValue 
    Else 
        Property_Name = field_vValue 
    End If 
End Property 

' You could use either Let or Set
Property Let Property_Name(vData As Variant) 
    field_vValue = rData 
End Property 

Property Set Property_Name(vData As Variant) 
    Set field_vValue = vData 
End Property 

Remember

If you are using an Object data type then you must use Set instead of Let.

Public Variables

Public variables declare the properties for the object.
When your properties are defined Public they are automatically available as properties of the class.
Any variables declared inside a class (even those declared as public) are not available outside of the class.
This is particularly relevant to Worksheet and Userform class modules.

Public property_sName As String 
Public property_dbHourlyRate As Double 

Both these variables are declared as Public and therefore automatically become properties of this class.

Disadvantages of Public Variables

Class has no way of knowing if an outside process has changed the value.
There is no way of performing any data validation.
There is no way of creating read-only or write-only properties.

Alternative to Public Variables

The alternative is to use fields with property procedures instead.
Property Get - used to retrieve a property of a class. Get the value of scalar of object property.
Property Let - used to assign the value to a property of a class. Set the value of scalar property.
Property Set - used to assign an object to a property of a class. Set a reference for object property.

Property Get

Used to retrieve a property of a class.

This is exactly the same as just using a private variable.

Public Property Get Property_Department() As String 
   Property_Department = field_sDepartment 
End Property 

The following line gets the department by calling the property Get procedure

Call MsgBox(oEmployee.Property_Department) 

Public Property Get Property_HoursPerWeek() As Double 
   Property_HoursPerWeek = field_dbNormalHours + field_dbOvertimeHours 
End Property 

Returns a property object reference

Public Property Get Property_SomeThing() As Object 
   Set Property_SomeThing = field_oObject 
End Property 

Property Let

used to change the (or assign) the value of a class property.
This statement lets you define a procedure that assigns a value to a property.
If you want to have full control when a property is initialised you can define the property using a Property Let procedure.

Public Property Let Property_One() 
End Property 

Passing Parameters

It is possible to pass parameters although not best practice.

Department

Public Property Let Property_Department(sDepartment As String) 
     If (sDepartment <> "Sales") And (sDepartment <> "Research") Then 
         Err.Raise "This is not a valid department name" 
         Exit Property 
     End If 
     
     field_sDepartment = sDepartment 
End Property 

The following line assigns a department by calling the Property Let procedure

objEmployee.Property_Department = "Sales" 

Hours Per Week

Public Property Let Property_HoursPerWeek(dbHoursPerWeek As Double) 
   field_dbNormalHours = WorksheetFunction.Min(35, field_dbHoursPerWeek) 
   field_dbOvertimeHours = WorksheetFunction.Max(0, field_dbHoursPerWeek - 35) 
End Property 

Write Only Properties

These are not used very often but it is possible to create jus a write-only property
Just provide a property let procedure (no property set)

Property Set

Used to assign (or change) an object assigned to a property.
Creates a reference to an object
This can be used to set a property to an object.
You can't define a property set inside another function or subroutine.

Private field_oEmployee As Object 

Public Property Set Property_InitialiseObject(oObject As Object) 
   Set field_oEmployee = oObject 
End Property 

You can only use a Property Set procedure on the left side of an object reference

objEmployee.Property_InitialiseObject = oEmployee 

Read Only Properties

It is very common to have read-only properties but not write-only.
A large number of the properties are typically read-only.

This property only has a Property Get (no Property Let) which means that it is read-only.

Public Property Get Property_NormalHours() As Double 
   Property_NormalHours = field_dbNormalHours 
End Property 

This property only has a Property Get (no Property Let) which means that it is read-only.

Public Property Get Property_OvertimeHours() As Double 
   Property_OvertimeHours = field_dbOvertimeHours 
End Property 

A lot of the object model classes contain a lot of properties that cannot be set.
If a property cannot be set, it is read-only as opposed to read-write).
Many properties have built in constants that should be used for their values.

Using a Function Instead

You can create functions that can be treated like a read-only property

Write Only Properties

When the value of a property can only be changed outside the class it is called a write-only property.
This is rarely seen or used.
There may also be times when you want the client to change a value but not to retrieve it.
This rarely makes sense though.
When a write-only property is required a method is often used instead.

Exit Property

This can be used to cause an immediate exit from the property
You can use any number in a property statement.

Private pAge As Integer 

Public Property Let Age(ByVal NewAge As Integer) 
    If (NewAge < 0) Then 
        MsgBox "Error: Age cannot be negative!", vbCritical 
        
        Exit Property 
    End If 
    
    pAge = NewAge 
End Property 

Public Property Get Age() As Integer 
    Age = pAge 
End Property