VB Script Properties

 VBScript Classes
 Creating Classes

Classes aren't a new concept in scripting. JavaScript, JScript, and other scripting languages have supported classes or similar elements for years. However, VBScript 5.0 is the first version of VBScript to support classes.

To use classes in your VBScript code, you first need to obtain VBScript 5.0 by downloading the appropriate self-executable file from the Microsoft Developer Network (MSDN) Web site (http://msdn.microsoft.com/scripting) or by installing Microsoft Internet Explorer (IE) 5.0. Then you need to understand what a VBScript class is and learn how to declare, define, initialize, and instantiate a class.

VBScript Classes 

VBScript 5.0 supports two types of objects: COM objects and Class objects (typically referred to as simply classes). VBScript COM objects have basic subtypes, such as an Integer or String. VBScript classes have an abstract subtype that encapsulates data and the functions to work with that data. You can think of a VBScript class as having a souped-up subtype that provides you with more computing power and flexibility. (Other differences exist between these two types of objects. For more information, see the Web-exclusive sidebar "How VBScript Classes and COM Objects Differ" on the Win32 Scripting Journal Web site at http://www.winntmag.com/ newsletter/scripting.

You can use classes to describe complex data structures. For example, if your application tracks customers and orders, you can define two classes for them, each with a unique set of internal data (typically called properties) and functions (typically called methods). You can then manage customers and orders as if they were native VBScript subtypes. More important, because you assign a class its properties and methods (i.e., its programming interface), you have an object-oriented tool to improve VBScript applications.

Declaring a Class 

You use the Class statement to declare a class. This statement's syntax is:

Class name
' Properties and methods go here.
End Class
where name is the name you give that class. You declare the properties and methods for your class between the Class and End Class clauses.

For example, suppose you want to create the VBScript class FileList, which Listing 1 contains. This class manages those files in a folder that meet a filename specification that you provide. You create this class by first specifying the keyword Class followed by the class' name Class FileList. Next, you declare the class' properties and methods. FileList has two properties (FileSpec and FolderPath) and one method (Search).

Declaring the FileSpec Property 

The FileSpec property holds the filename specification. For example, the filename specification might be C:\*.*. You want users to be able to freely read and write values to this property, so you declare FileSpec as an external, or public, variable with the Public statement

Public FileSpec

You can use a public variable in any script, not just the script in which you created the variable. However, if you use a public variable, you have no control over the value that users assign to the variable and no control over the value that the variable returns. Thus, you can't use public variables to hold values that you need to validate.

Declaring the FolderPath Property

The FolderPath property holds the full path to the folder containing the files. After a user sets a folder path, you need to validate that the folder exists, which means you can't use a public variable. Instead, you need to store the folder path in an internal, or private, variable and use two public property procedures to read and write to that variable. (Public property procedures are wrappers that hide the code that gets and sets the values of private variables.)

Prefixing a private variable with the m_ string is a common scripting convention. For example, the private variable for the FolderPath property is m_folderPath. To declare m_folderPath, you use the Private statement

Private m_folderPath

Procedures and variables that have the Private qualifier aren't visible outside the class. In addition, private variables apply only to the script in which you created them.

After you declare m_folderPath, you need to declare the two public property procedures that you'll use to read and write to that variable. The first procedure to declare is the Property Get procedure, which returns the values of properties. The second procedure is the Property Let procedure, which assigns values to properties.

To declare the Property Get procedure, you use the Property Get statement

Public Property Get FolderPath
    FolderPath = m_folderPath
End Property
where FolderPath is the name of that procedure. By including the Public statement with the Property Get statement, you're making the value that the FolderPath procedure returns available for public reading. Thus, by assigning FolderPath to m_folderPath, you make the value of m_folderPath available for public reading.


Post a Comment