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Objects and Classes
Traditional programming is made up of a collection of subroutines and functions that are typically
processed in a sequential or looping manner. In contrast, object oriented programming is a different
programming methodology where a program is viewed as being composed of a collection of individual
objects. These objects process data and can interact with other objects directly without having to be
explicitly programmed to do so. The advantages claimed by object-oriented program include code
reusability, rapid deployment of large-scale complex tasks, and ease of use/debugging. Today, object-
-
oriented programming is widely used and is supported with both programming languages (e.g. VB.NET,
C++, Visual C++) and operating systems (e.g. Microsoft’s .NET architecture). Object-oriented
programming has also become popular within scripting languages, such as VBScript. Beginning with
VBScript 5.0, developers have been able to use user-defined Classes.
The key concepts with object-oriented programming include:
• Class
The class is the highest level that defines a unit (set) of data and its behavior. Classes form the
basis for modularity and structure in an object-oriented program. The class should sufficiently
describe the set of data, and the code for a class should be contained within it and be self-
sufficient (except for operating system support). While the terms classes and objects often get
used interchangeably, classes describe the structure of objects. One way to think of a class is
that it is a container for code. It can also be viewed as a template for an object. When a class is
declared (instantiated) by the Set statement, it then becomes an object and memory is allocated
for it.
• Object
An object is an in-memory instance of a class. In computer science terms, it is a run-time
e
manifestation (instantiation) of a particular exemplar of a class. Each object has its own data,
but the code within a class can be shared (for efficiency). Programs generally have multiple
objects. Multiple copies (objects) of a given class can be created. Objects are temporary, i.e.
they can be created and removed at will, depending on the programming needs.
• Encapsulation
Encapsulation wraps the data and functions into a single unit, ensuring that the object can be
changed only through established interfaces. Encapsulation is sometimes referred to as
information hiding. Some of these common interfaces are:
o Fields
Fields are simply public variables stored within the object, as defined by the class. These
variables store items of information about an object.
o Properties
Properties, like fields, also store items of information on an object. But Properties use
Property procedures to control how values are set or returned. VBScript has two primary
Property procedures; Let and Get. The Get property procedure retrieves a Property
rty
value, while the Let Property procedure assigns a value to the property value. A third
Property procedure Set is used with an Object inside of the Class block.
o Methods
Methods are a collection of subroutines (Sub) and function procedures (Function)
)
declared within a class.
o Events
An event is a message sent by an object announcing that something important has
happened.
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Access of an object’s methods, properties and fields are made by referring to the object,
followed by a period, then the particular method, property or field of interest. E.g.
Object.Method
Object.Property
Object.Property.Item
• Dynamism
Dynamism relates to the method of allocating computer resources and definition resources
required to run an object-oriented program. There are different types, but VBScript used late-
-
bound (late-binding) dynamic typing. This means that the VBScript engine will make the object
type determination at runtime and allocate sufficient memory at that time. Note that VBScript
and VB.NET are slightly different in their approach to dynamism, and therefore they can declare
some variables and objects in different manners (although many forms of declaration are the
same).
• Outlet Connections
At times, Objects will connect together and this connection needs to be defined. With IWS, an
example of a connection would be between a VBScript object (e.g. ADODB) and a Database
Provider (a Provider is a front-end to a database). This connection needs to be defined, and
then the connection string (of parameters) between the objects gets defined. When the need for
the connection is finished, the connection should be closed.
While a full treatment of object-oriented programming is beyond the scope of these materials, the
fundamental concepts of Objects and Classes are important to understand. VBScript supports COM-
based Objects (Component Object Module, a Microsoft standard) such as the ActiveX controls,
ADO.NET, FileSystemObject, and Microsoft Office Automation objects. VBScript also supports user-
defined classes, or Class Objects.
VBScript COM objects and VBScript Class objects differ from each other in several important respects.
These differences lead to each type of object having its unique strengths:
•
VBScript classes are more flexible than VBScript COM objects. Class Objects have an abstract
subtype that encapsulates the data you want and the functions you need to work with that data.
VBScript COM objects have only basic subtypes (integer or string).
•
VBScript classes are slightly more efficient than COM objects. The VBScript parser can execute
the classes' code directly instead of asking the COM object to execute a method.
•
COM objects are binary modules. VBScript classes are ASCII files.
•
You can use any scripting language to write COM objects. You can only use VBScript to write
VBScript classes.
•
You can use COM objects from within any development environment that supports COM
automation. VBScript classes can only be used within development and runtime environments
s
that support VBScript (e.g IWS and Microsoft Internet Explorer).
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VBScript Object Commands
VBScript includes several Functions and Statements that can be used to access objects, including their
methods and properties. There are a large variety of objects available to VBSript, including user-
defined objects, intrinsic objects and extrinsic objects.
VBScript Object Functions
Function
Description
CreateObject
Creates and returns a reference to an Automation object
GetObject
Returns a reference to an Automation object from a file
IsObject
Returns a Boolean value indicating whether an expression references a valid Automation
object.
Object Statements
Error Handling Statements
Statement
Description
On Error
Enables or disables error-handling
Object & Collection Summary
Objects & Collections
Description
Debug
The Debug object is an intrinsic global object that can send an output to a script
debugger, such as the Microsoft Script Debugger.
Dictionary
An associative array that can store any type of data. Data is accessed by a key.
Drive
An object that refers to a specific Drive
Drives
A collection of Drive objects.
Err
Contains information about the last run-time error. Accepts the Raise and Clear
ar
methods for generating and clearing run-time errors.
File
An object that refers to a specific File
Files
A collection of File objects.
FileSystemObject
An object model used to access the Windows file system
Folder
An object that refers to a specific Folder
Folders
A collection of Folder objects.
Match
Provides access to the read-only properties of a regular expression match.
Matches
Collection of regular expression Match objects.
RegExp
Provides simple regular expression support.
Submatches
A collection of regular expression submatch strings.
TextStream
An object that refers to a text File
Statement
Description
Class
Declares the name of a class, as well as a definition of the variables, properties, and
methods that comprise the class
Exit Property
Forces an exit from inside a Property Set function.
For Each…Next
Repeats a group of statements for each element in an array or a collection.
Property Get
Declares, in a Class block, the name, arguments, and code that form the body of a Property
procedure that gets (returns) the value of a property
Property Let
Declares, in a Class block, the name, arguments, and code that form the body of a Property
procedure that assigns (sets) the value of a property
Property Set
Sets a reference to an object
Set
Assigns an object reference to a variable or property, or associates a procedure reference
with an event. Usually used to instantiate an object.
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VBScript User-Defined Class Objects
To define a user-defined Class Object, you use the Class statement to declare a class. The End Class
ass
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
a
variable. With VBScript, this is the only time the New keyword is used (i.e. to instantiate a user-defined
ed
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.
ts.
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
ible
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
ment
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.
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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
or
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
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End Property
End Class
Rem Below is the VBScript code
Dim objFSO
‘ Declare variables and objects
Dim objFilePointer, cur_file
Set objFSO = CreateObject(“Scripting.FileSystemObject”)
‘ Instantiate the COM object
Set objFilePointer = New FileSpec
‘ Instantiate the Class Object
objFilePointer.FileName = “Myfile.mdb” ‘ Assigns “Myfile.mdb” as the file name
cur_file = objFilePointer.FileName
‘ Retrieves the current file name “Myfile.MDB”
Set objFilePointer.FSO = objFSO
‘ Assigns an Object to the Property
Set objFilePointer = Nothing
‘ Keyword Nothing releases the object memory
A couple notes on the example above. The CreateObject command is used to instantiate an Object
that is known at the system level (e.g. a COM object). Also, so far this example only shows how to
assign and retrieve property values. It is generally the Method(s) that control the action an object
performs, not the properties.
A Property can be made read-only by only providing a Property Get procedure, or by declaring the
Property Let procedure as Private instead of Public. A Property can be made write-only by only
providing the Property Let procedure, or by declaring the Property Get procedure as Private instead of
Public.
Class Methods are really just Functions and Subroutines inside of a Class block. These functions and
d
subroutines can be either Private or Public. If they are public, they will be accessible to a VBScript code
segment outside of the Class block by referencing the obj.Method. If they are private, they will only be
available to code within the Class block.
An example of Class Methods is as follows:
Class FileSpec
Private master_file
Private master_FSO
Private master_file
Private Sub Class_Initialize
‘ Class Object initialization code
‘ code goes here
End Sub
Private Sub Class_Terminate
‘ Class Object termination code
‘ code goes here
End Sub
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
End Property
Public Sub Delete
‘Method to delete the master file
master_FSO.DeleteFile (master_file)
End Sub
End Class
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Rem Below is the VBScript code
Dim objFSO
‘ Declare variables and objects
Dim objFilePointer, cur_file
Set objFSO = CreateObject(“Scripting.FileSystemObject”)
‘ Instantiate the COM object
Set objFilePointer = New FileSpec
‘ Instantiate the Class Object
objFilePointer.FileName = “Myfile.mdb” ‘ Assigns “Myfile.mdb” as the file name
cur_file = objFilePointer.FileName
‘ Retrieves the current file name “Myfile.MDB”
Set objFilePointer.FSO = objFSO
‘ Assigns an Object to the Property
objFilePointer.Delete
‘ Executes a Method to delete a file
Set objFilePointer = Nothing
‘ Keyword Nothing releases the object memory
VBScript Class Objects automatically supports two type of Class Events; Class_Initialize and
Class_Terminate Events. The code inside the Class_Initialize event executes once when an Object
based on the Class is first instantiated. Any code put in this event is optional, and is typically used for
initialization. Code inside the Class_Terminate event executes once just before the Object based on the
Class is destroyed (i.e. Set to Nothing, or the Object goes out of scope). Usage is as follows:
Class FileSpec
Private master_file
Private master_FSO
Private master_file
Private Sub Class_Initialize
‘ Class Object initialization code
‘ code goes here
End Sub
Private Sub Class_Terminate
‘ Class Object termination code
‘ code goes here
End Sub
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
End Property
End Class
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VBScript Objects and Collections
VBScript has certain Objects and Collections that are inherent with VBScript. These include:
• Debug
• Err Object
• Match Object & Matches Collections
• Scripting Dictionary Object
• Scripting FileSystemObject
o Drive Object
o File Object
o FileSystemObject Collections
o Folder Object
• Regular Expression Object & Submatches Collection
• TextStream Object
VBScript Implicit Objects and Collections
Objects & Collections
Description
Class Object
Declares the name of a class, as well as a definition of the variables, properties,
and methods that comprise the class
Debug
The Debug object is an intrinsic global object that can send an output to a script
debugger, such as the Microsoft Script Debugger.
Err
Contains information about the last run-time error. Accepts the Raise and Clear
ear
methods for generating and clearing run-time errors.
Match Object
Dictionary
An associative array that can store any type of data. Data is accessed by a key.
Matches Collection
RegExp Object
SubMatches Collection
Object & Collection Summary
Objects & Collections
Description
Drive
An object that refers to a specific Drive
Drives
A collection of Drive objects.
File
An object that refers to a specific File
Files
A collection of File objects.
FileSystemObject
An object model used to access the Windows file system
Folder
An object that refers to a specific Folder
Folders
A collection of Folder objects.
Match
Provides access to the read-only properties of a regular expression match.
Matches
Collection of regular expression Match objects.
RegExp
Provides simple regular expression support.
Submatches
A collection of regular expression submatch strings.
TextStream
An object that refers to a text File
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Err Object
The VBScript Err object contains information about run-time errors.
.
Err Object Properties
Properties
Description
Description
The descriptive string associated with an error.
HelpContext
A context ID for a topic in a Windows help file.
HelpFile
A fully qualified path to a Windows help file.
Number
A numeric value identifying an error.
Source
The name of the object or application that originally generated the error.
Err Object Methods
Properties
Description
Clear
Clears all property settings.
Raise
Generates a run-time error.
The properties of the Err object are set by the generator of an error-Visual Basic, an Automation object,
t,
or the VBScript programmer.
The default property of the Err object is Number. Err.Number contains an integer and can be used by
y
an Automation object to return an SCODE.
When a run-time error occurs, the properties of the Err object are filled with information that uniquely
ly
identifies the error and information that can be used to handle it. To generate a run-time error in your
code, use the VBScript Err Object Raise Method. The Err object's properties are reset to zero or zero-
-
length strings ("") after an On Error Resume Next statement. The VBScript Err Object Clear Method
can be used to explicitly reset Err.
The Err object is an intrinsic object with global scope-there is no need to create an instance of it in your
r
code.
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Scripting Dictionary Object
A dictionary object is part of the Scripting type library. The dictionary object is a special type of an array
which stores a data item that is associated with a unique key. The key, which is usually a number or a
string, is used to retrieve an individual item. You can use a Dictionary when you need to access random
elements frequently or need to access information contained in the array based on its value, not
position.
The Dictionary object has both Methods and Properties that can be used to manipulate the Dictionary.
Dictionary Methods
Method
Description
Add
Adds a key and item pair
Exists
Indicates if a specific key exists
Items
Returns an array containing all items in a Dictionary object
Keys
Returns an array containing all keys in a Dictionary object
Remove
Removes a key, item pair
RemoveAll
Removes all key, item pairs
Dictionary Properties
Method
Description
CompareMode The comparison mode for string keys
Count
The number of items in a Dictionary object
Item
An item for a key
Key
A key
The following code creates a Dictionary object and adds items and keys:
:
Dim d 'Create a variable
Set d = CreateObject("Scripting.Dictionary")
d.Add "a", "Athens"
'Add some keys and items
d.Add "b", "Belgrade"
d.Add "c", "Cairo"
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