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Tip of the Day
Language: VB5,VB6
Expertise: Intermediate
Jun 3, 2000

Use Enums to restrict parameter values in Classes, ActiveX Controls and DLLs

By defining a Public Enum and using it as a parameter type to your property or function, client applications are presented with a drop down list of values to use. Unfortunately, VB doesn't restrict the client from passing something other than one of your enumerated values, since all Enums are Longs. However, you can use hidden Enum members to check the value of the parameter.

Public Enum My_Enum
    [_MY_ENUM_LOWER] = 0
    ENUM_VALUE_1
    ENUM_VALUE_2
    ' ...
    [_MY_ENUM_UPPER]
End Enum

Private m_MyValue as My_Enum

Public Property Let MyValue(ByVal New_Value as My_Enum)
    If New_Value <= [_MY_ENUM_LOWER] Or New_Value >= [_MY_ENUM_UPPER] Then
        Err.Raise 5
        'Invalid procédure call or argument, a standard VB error
        '  you may want to add vbObjectError for DLLs
    Else
        m_MyValue = New_Value
    EndIf
End Property
By using an underscore in front of the Enum member, it is hidden from both the drop down list and the Object Browser (unless you select the Show Hidden Members option). The underscore as first character for identifiers is illegal in VB, so you have to enclose the name in square brackets. UPDATE: Scott B. Kardos had us notice that the hidden enums are still Public and binary compatability must be broken if ENUM_VALUE_3 is added. Since the hidden enums are only used for limit checking in the class, Scott recommends the following changes which allow for binary compatability:

Public Enum My_Enum
    ENUM_VALUE_1
    ENUM_VALUE_2
    ENUM_VALUE_3
    ' ...
End Enum

Private Enum My_Enum_Limits
    MY_ENUM_LOWER = ENUM_VALUE_1
    MY_ENUM_UPPER = ENUM_VALUE_3
End Enum

Private m_MyValue As My_Enum

Public Property Let MyValue(ByVal New_Value As My_Enum)
    If New_Value < MY_ENUM_LOWER Or New_Value > MY_ENUM_UPPER Then
        Err.Raise 5
        'Invalid procédure call or argument, a standard VB error
        '  you may want to add vbObjectError for DLLs
    Else
        m_MyValue = New_Value
    End If
End Property

Scott
Glen McLeod
 
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