ACC97: IsMissing() Returns False for Non-Variant Data Types

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When you pass optional arguments that are of a data type other thanVariant to a Visual Basic for Applications function, the IsMissing()function returns a value of False for those Optional arguments.

This is perceived as a problem because IsMissing() will return False forany non-Variant data type, regardless of whether an optional argument ispassed or not.
The IsMissing() function is used to detect whether or not optional Variantarguments have been provided in calling a procedure. It will always returnFalse for non-Variant data types.
This article assumes that you are familiar with Visual Basic forApplications and with creating Microsoft Access applications using theprogramming tools provided with Microsoft Access. For more informationabout Visual Basic for Applications, please refer to the "BuildingApplications with Microsoft Access 97" manual.

Initialize your optional non-Variant variables with a default value whendefining the function. The following is an example declaration of afunction, including the initialization of an Optional string argument witha default value of "Empty String":
Public Function Test(Optional str1 As String = "Empty String", _   Optional var2 As Variant)				
Optional arguments are used in procedures to indicate that an argument isnot required. Although the IsMissing() function always returns False fornon-variant data types, it is important to learn what values are returnedif the optional argument is not specified when calling the procedure. Thefollowing list summarizes what values are returned if an optional argumentis not specified or an optional argument is not initialized with a defaultvalue:
   Data Type           Value returned by Default   ---------           ---------------------------------   String              "" (returns a zero length string)   Variant             Error 448   Integer             0   Single              0   Long                0   Double              0   Currency            0   Date                12:00:00 AM   Byte                0   Boolean             False   Object              (Run-time error 91: "Object variable or With block                       variable not set")				

Steps to Reproduce Behavior

The following example demonstrates what the IsMissing() function willreturn, given an optional String and optional Variant arguments. In orderto assure the optional String argument won't return a zero-length string,you will want to replace the function definition with the one noted in the"Resolution" section of this article:
  1. Create a module and type the following line in the Declarations section if it is not already there:
    Option Explicit						
  2. Type the following procedure:
    Public Function Test1(Optional str1 as String, _                    Optional var2 as Variant)   Debug.Print "IsMissing(str1) = " & IsMissing(str1)   Debug.Print "IsMissing(var2) = " & IsMissing(var2)   Debug.Print str1   Debug.Print var2End Function					
  3. To test this function, type the following line in the Debug window,and then press ENTER.
    Note that the following information is returned:
    IsMissing(str1) = False
    IsMissing(var2) = True

    Error 448
For more information about using optional arguments in procedures, searchthe Help Index for "Optional Keyword."

For more information about understanding optional arguments, search theHelp Index for "Optional arguments, Understanding Named Arguments andOptional Arguments."

For more information about using the IsMissing() function, search the HelpIndex for "IsMissing Function."
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Article ID: 162534 - Last Review: 01/06/2015 01:52:14 - Revision: 3.3

  • Microsoft Access 97 Standard Edition
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