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

• Microsoft Access 97

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

This article assumes that you are familiar with Visual Basic for Applications and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Visual Basic for Applications, please refer to the "Building Applications with Microsoft Access 97" manual.

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:

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 var2
   
         End Function
   
   

3. To test this function, type the following line in the Debug window, and then press ENTER.

          ?Test1()
   

   Note that the following information is returned:

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

          Error 448
   

REFERENCES

For more information about using optional arguments in procedures, search the Help Index for "Optional Keyword."

For more information about understanding optional arguments, search the Help Index for "Optional arguments, Understanding Named Arguments and Optional Arguments."

For more information about using the IsMissing() function, search the Help Index for "IsMissing Function."