Microsoft Access Constants — Overview

See Also

General

A constant represents a numeric or string value that doesn't change. You can use constants to improve the readability of your Visual Basic code and to make your code easier to maintain. In addition, the use of intrinsic constants ensures that code will continue to work even if the underlying values that the constants represent are changed in later releases of Microsoft Access.

Microsoft Access supports three types of constants:

• Symbolic constants, which you create by using the Const statement and use in modules.
  
  
• Intrinsic constants, which are part of Microsoft Access or a referenced library.
  
  
• System-defined constants: True, False, and Null.

Symbolic Constants

Often, you'll use the same values repeatedly in your code, or you'll find that the code depends on certain numbers that have no obvious meaning. In these cases, you can make the code much easier to read and maintain by using symbolic or user-defined constants, which enable you to use a meaningful name in place of a number or string.

Once you have created a constant by using the Const statement, you can't modify it or assign a new value to it. You also can't create a constant that has the same name as an intrinsic constant.

The following examples show some of the ways you can use the Const statement to declare numeric and string constants:

Const conPI = 3.14159265                ' Pi equals this number.
Const conPI2 = conPI * 2                ' A constant used to create another.
Const conVersion = "Version 7.0"        ' Declare a string constant.

Intrinsic Constants

In addition to the constants you declare with the Const statement, Microsoft Access automatically declares a number of intrinsic constants and provides access to the Visual Basic for Applications (VBA) constants, and ActiveX Data Objects (ADO) constants. You can also use constants in other referenced object libraries. For more information on adding references see Setting References to Type Libraries.

Any intrinsic constant can be used in a macro or Visual Basic. These constants are available at all times. The specific built-in constants used with a particular function, method, or property are described in the Help topic for that function, method, or property.

Note   You can use the Object Browser to view lists of intrinsic constants from all available object libraries.

Intrinsic constants have a two letter prefix identifying the object library that defines the constant. Constants from the Microsoft Access library are prefaced with "ac"; constants from the ADO library are prefaced with "ad"; and constants from the Visual Basic library are prefaced with "vb". For example:

• acForm
  
  
• adAddNew
  
  
• vbCurrency

Note   Because the values represented by the intrinsic constants may change in future versions of Microsoft Access, you should use the constants instead of their actual values. You can, however, display the actual value of a constant by choosing the constant in the Object Browser or by typing ?constantname in the Immediate window.

You can use intrinsic constants wherever you can use symbolic, or user-defined constants, including in expressions. The following example shows how you might use the intrinsic constant vbCurrency to determine whether the variable varNum is a Variant for which the VarType function returns 6 (Currency):

Dim varNum As Variant
If VarType(varNum) = vbCurrency Then
    Debug.Print "varNum contains Currency data."
Else
    Debug.Print "varNum doesn't contain Currency data."
End If

There are several categories of intrinsic constants. For a list of the intrinsic constants in a particular category, click the category below:

• The action constants
  
  
• The ADO constants
  
  
• The DAO constants
  
  
• The event procedure constants
  
  
• The Keycode constants
  
  
• The miscellaneous constants
  
  
• The RunCommand method constants (these are a subset of the action constants)
  
  
• The security constants
  
  
• The Visual Basic constants
  
  
• The VarType function constants

System-Defined Constants

You can use the system-defined constants True, False, and Null anywhere in Microsoft Access. For example, you can use True in the following macro condition expression. The condition is met if the Visible property setting for the Employees form equals True.

Forms!Employees.Visible = True

You can use the constant Null anywhere in Microsoft Access. For example, you can use Null to set the DefaultValue property for a form control by using the following expression:

=Null