ACC2000: IsMissing() Returns False for Non-Variant Data Types

ID: Q208401

The information in this article applies to:

Microsoft Access 2000

SYMPTOMS

Advanced: Requires expert coding, interoperability, and multiuser skills.

When you pass optional arguments that are of a data type other than Variant 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 for any non-Variant data type, regardless of whether an optional argument is passed or not.

CAUSE

The IsMissing() function is used to detect whether or not optional Variant arguments have been provided in calling a procedure. It will always return False for non-Variant data types.

RESOLUTION

Microsoft provides programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes that you are familiar with the programming language being demonstrated and the tools used to create and debug procedures. Microsoft support professionals can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific needs. If you have limited programming experience, you may want to contact a Microsoft Certified Solution Provider or the Microsoft fee-based consulting line at (800) 936-5200. For more information about Microsoft Certified Solution Providers, please see the following page on the World Wide Web:

http://www.microsoft.com/mcsp/

For more information about the support options available from Microsoft, please see the following page on the World Wide Web:

http://www.microsoft.com/support/supportnet/overview/overview.asp

Initialize your optional non-Variant variables with a default value when defining the function. The following is an example declaration of a function, including the initialization of an optional string argument with a default value of "Empty String":


Public Function Test(Optional str1 As String = "Empty String", _
                     Optional var2 As Variant)

MORE INFORMATION

Optional arguments are used in procedures to indicate that an argument is not required. Although the IsMissing() function always returns False for non-Variant data types, it is important to learn what values are returned if the optional argument is not specified when calling the procedure. The following list summarizes what values are returned if an optional argument is not specified or an optional argument is not initialized with a default value:


   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 will return, given an optional String and optional Variant arguments. In order to 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:

Create a module and type the following line in the Declarations section if it is not already there:


Option Explicit

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 var2
End Function

To test this function, type the following line in the Immediate Window, and then press ENTER:


?Test1()

You should see the following information returned:

IsMissing(str1) = False
IsMissing(var2) = True
Error 448

REFERENCES

For more information about the IsMissing function, in the Visual Basic Editor, click Microsoft Visual Basic Help on the Help menu, type "Ismissing" in the Office Assistant or the Answer Wizard, and then click Search to view the topic.

Additional query words: type cast form report object ParamArray null prb


Keywords          : kbenv kbprg kbdta 
Version           : WINDOWS:2000
Platform          : WINDOWS 
Issue type        : kbprb

Last Reviewed: July 6, 1999