The Internet Encyclopedia (Volume 3)

P1: JDT

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626 VISUALBASICSCRIPTINGEDITION(VBSCRIPT)

follow such conventions, doing so tends to improve read-

ability and sharability of code. There are several recog-

nized coding conventions used by VBScript programmers

(Microsoft, 2002c). Two naming patterns are commonly

used for named constants. One pattern specifies all upper

case for names of constants. When using this pattern, indi-

vidual words within the name of a constant are delineated

using an underscore (thus, “MYCONSTANTNAME”).

An alternative approach is to prefix names of constants

with “con” and use a mixed case pattern where the first

letter of embedded words is upper case (thus, “conMy-

ConstantName”). This latter approach follows from vari-

able naming conventions, where a three-letter prefix is

used to indicate the variant subtype of the variable (i.e.,

strMyStringVariable). Sometimes an additional character

(“s”) is prefixed to names of variables having script-level

scope (thus, sintMyIntegerVariable). User-defined subrou-

tine and function names are also typically mixed case and

usually begin with a verb describing what the procedure

does—e.g., “Sub FetchEnvironmentValues ()”.

Lines prefaced by either the string “REM” or a sin-

gle apostrophe are interpreted in VBScript as in-program

comments and are not processed by the VBScript inter-

preter. A single apostrophe following a VBScript state-

ment is also interpreted as the start of a comment. To use

the REM key word on the same line following a VBScript

statement, the REM key word must be preceded by a

colon. (A colon is the delimiter used to allow multiple VB-

Script statements on a single line.) All text following either

the key word REM or a single apostrophe is ignored by the

scripting engine to the end of the current line. In-program

comments not only should be descriptive, but also should

focus on what rather than how, in order to minimize com-

ment maintenance. Thus each procedure or subprocedure

should begin with an extended comment describing the

purpose of the procedure, its required input(s), any out-

puts, any assumptions about inputs and outputs, and its

return and/or the global variables effected. A minimum

should be said about how it accomplishes these tasks since

that information is typically more volatile.

When scripts are created, indenting should be used to

show flow control and logical structure of the code. Thus

blocks of code within an If... Then... Else statement (and

similar flow control statements) will be indented relative

to the If and Else lines themselves. To conserve screen

space and allow for nesting of code blocks, a typical indent

is only three to five spaces.

Finally, though not required by the language, many

programmers find it useful to declare (using Dim state-

ments) all variables and named constants before use.

Starting a script off with the line “Option Explicit” re-

quires that all variables and named constants used in that

script be declared before first use. If a typo in a name is

made in the code, having option explicit in effect will im-

mediately highlight the error, which can greatly facilitate

code debugging.

OBJECTS IN VBSCRIPT

Use of Objects in VBScript

Objects are abstract structures used in high-level pro-

gramming languages to encapsulate discrete modules of

code and associated data structures. They enable modular

implementation and reuse of code and facilitate collab-

orative development of advanced applications. When the

archetype of a programming object, called an object class,

is written, standard types of programming interfaces and

class behaviors are defined that allow instances of objects

created from the class definition to be used easily and

reliably in many different programs. Instead of writing

their own code to perform common tasks, programmers

can use classes as templates to instantiate (create) object

instances for use in their programs. Programmers imple-

menting a predefined class can treat an object instance

created from that class like a “black box.” A programmer

using an object class does not have to understand how the

class works internally. All he or she has to understand is

how to use the interfaces and behaviors of the class. In-

terfaces and behaviors of external object classes invoked

using VBScript are described in terms of properties, meth-

ods, and events. Object classes written in more advanced

object-oriented programming languages (e.g., C++) may

also exhibit polymorphism and inheritance. These latter

two behaviors are not supported in VBScript; however,

they are supported in Visual Basic.NET.

VBScript provides support for the use of registered

ActiveX object classes, also referred to as object linking

and embedding server objects and COM or COM+ objects.

While VBScript provides a language intrinsic CreateOb-

ject method for instantiating a new instance of an object

and a language intrinsic GetObject method for creating

additional references to an already instantiated object, it

is usually better when possible to invoke equivalent meth-

ods built into an application-specific object model pro-

vided by the scripting host being used. This allows the

scripting host to track the use of the object and resources

consumed by it and integrate the object into the host ap-

plication environment. Thus, when a script is written for

the WSH, the preferred syntax to create an instance of an

Active Data Object (ADO) Recordset object would be

Set objMyRecordset = WScript.CreateObject

("ADODB.Recordset")

Note that the string argument of the WScript object’s Cre-

ateObject method is the ProgID of the object, typically

consisting of the library or ActiveX server name and the

type name (class name) of the object being instantiated.

ProgIDs are maintained in the HKEYCLASSESROOT

key of the host system’s registry. Note the use of the “Set”

key word in the statement that assigns the reference for

the object to the variable objMyRecordset. This is how

VBScript knows you are assigning an object reference to

a variable. When finished with an object, you should use

the “Nothing” key word to free resources assigned to the

variable; thus,

Set objMyRecordset = Nothing

Note also that VBScript uses dot syntax for refer-

encing an object’s methods and properties (includ-

ing collections of properties) and underscores syntax

for referencing an object’s events. Thus in this exam-

ple WScript.CreateObject() referenced the CreateObject