Microsoft KB Archive/162534

= ACC97: IsMissing Returns False for Non-Variant Data Types =

Article ID: 162534

Article Last Modified on 1/19/2007

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APPLIES TO


 * Microsoft Access 97 Standard Edition

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This article was previously published under Q162534



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



SYMPTOMS
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:   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 Debug window, and then press ENTER.

?Test1

Note that the following information is returned:

IsMissing(str1) = False

IsMissing(var2) = True

Error 448



