VBScript Reference Manual for InduSoft Web Studio

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VBScript Reference Manual InduSoft Web Studio VBScript User-Defined Class Objects To define a user-defined Class Object, you use the Class statement to declare a class. The End Class statement defines the termination of the Class. Together, these statements form a Class construct, or Class block. E.g. Class objName ‘ Place the Class variables, Properties and Methods here End Class In this syntax, objName is the name given to the Class Object. The class object name must follow standard VBScript variable naming conventions. Class Objects are usually declared in the variable definition sections. You can have multiple Class blocks in a single VBScript file, but each block must contain the Class …End Class statements. Classes cannot be nested. Once you have defined the Class Object, you need to create an instance of the Class, similar to how other objects are created. When the Class Object is instantiated, memory is allocated for the Class Object. The Set statement is used with the New keyword to assign an instance of the class to a variable. With VBScript, this is the only time the New keyword is used (i.e. to instantiate a user-defined Class). E.g. Dim MyObj Set MyObj = New objName The Object name MyObj is the Object variable name, and must follow standard VBScript variable naming conventions. The Object variable name is a reference (address) of the Object stored in memory, it is not the Object itself. Inside the Class block, any Class variables, Properties, Methods and Events can be defined by the developer. The developer does not have to use all of the capabilities of the Class construct, i.e. Classes can be created without Methods or Properties. The design of the Class Object is completely up to the developer. Class variables are created within the Class structure by using the Dim, Public, or Private statements. Variables defined within the Class structure by any of these statements must follow the standard VBScript variable naming conventions. Variables can be simple variables or arrays. E.g. Class className Dim var1, var2 Public var3, var4 Private var5, var6 End Class The choice of the Dim, Public, or Private statements determine whether the variable is accessible outside of the Class Object. Variables are public by default, i.e. they are accessible outside of the Class Object. Both the Dim and the Public statements create public variables, while the Private statement creates variables that are not public. As a general rule, it is good programming practice to make all Class variables private, since the developer will want to tightly control when these variables are changed. VBScript does not support Class-level Constants, i.e. named constants declared at the Class level. You cannot use the Const statement at the Class-level so that a constant can be used throughout a Class, but you can use the Const statement within a Property or Method. However, the constant will only have local scope within the Property or Method. 24 InduSoft, Ltd.
InduSoft Web Studio VBScript Reference Manual Class Object variables are accessible to VBScript code outside the Class through Class Properties. Class Properties “wrap” the Private variables of a Class. Inside the Class block, the Properties are defined by Property Get [|Let|Set] … End Property statement(s). For VBScript code outside the Class, the Property is accessed by referencing the Object Name.Property. There are different type of Class Properties, depending on whether the Class variable is to be read, written to, or the Class variable is itself a Class Object. These Properties can be declared Public or Private. Property Get The Property Get procedure is used to access (return) private variables inside of the Class structure that are used as a read-only Property, or the read portion of a read-write Property. For VBScript code outside the Class, this type of Class Object Property is generally assigned to a variable or used in a conditional expression. The Property Get procedure returns a value to the calling code, and is general not used with any arguments. [Note: VBScript will let you add arguments to the Property Get procedure, but if you do so you must add the additional argument to the corresponding Property Let or Property Set procedure, since Property Let/Property Set must have one more argument than the corresponding Property Get procedure. It is generally considered bad programming form to have arguments in the Property Get procedure]. Property Let The Property Let procedure is used to access (assign) private variables inside of the Class structure that are used as a write-only Property or are the write portion of a read-write Property. For VBScript code outside of the Class, this type of Class Object Property is usually assigned by a variable or a constant. Property Set The Property Set procedure is exclusively used when the Class Object needs to store Properties that are object-based instead of numeric, date, boolean or string subtype variables. Property Set replaces the Property Let procedure. While Property Set and Property Let are functionally similar, there are two key differences: 1. With the Property Set procedure, in the VBScript code segment (outside the Class block) you must use the syntax Set Object1.Property = Object2 This is because VBScript does not let you use the assignment operator (=) to assign objects without the Set command. 2. The Property Set procedure makes it clear that the Property is an object-based Property Example: Class FileSpec ‘ Define a Class block Private master_file Private master_FSO Public Property Let FileName(strName) ‘ Define a Public Property to assign the file name master_file = strName End Property Public Property Get FileName ‘ Define a Public Property to retrieve a file name FileName = master_file End Property Public Property Set FSO(m_FSO) ‘ Define a Public Property for an object Set master_FSO = m_FSO InduSoft, Ltd. 25
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