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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.
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.
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":
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 BehaviorThe 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:
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."
Article ID: 162534 - Last Review: January 6, 2015 - Revision: 3.3