Use Enums to restrict parameter values in Classes, ActiveX Controls and DLLs [ All Languages » VB »  Active-X] Total Hit ( 3008) Rate this article:     Poor     Excellent   Submit Your Question/Comment about this article Rating

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. Click here to copy the following block 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: Click here to copy the following block 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

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Submitted By : Nayan Patel  (Member Since : 5/26/2004 12:23:06 PM)
	Job Description : He is the moderator of this site and currently working as an independent consultant. He works with VB.net/ASP.net, SQL Server and other MS technologies. He is MCSD.net, MCDBA and MCSE. In his free time he likes to watch funny movies and doing oil painting.
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