Classes And Objects by George Hernandez

IUnknown Interface
Binding
Object Methods

IUnknown Interface

In addition to communicating with other components, a component must be able to create and destroy an object. All Microsoft COM components implement the creation and destruction of objects with the IUnknown interface. The IUnknown interface includes three functions.

QueryInterface

Asks the component if it supports a particular interface. In VB this function is accessed by binding an object variable to a particular class:

Dim ObjectVariableName as [New] ClassName

If you declare an object without using the New keyword, the object is never actually instantiated, unless the object variable is, at some point, Set = New ClassName.
If you declare an object using the New keyword, the object is actually instantiated as soon as a property or method of the object is used.
Many object variables can refer to the same instance of a class. Use the New keyword to make a new instance of the class.
The New keyword cannot be used to create new instances of intrinsic data types.
VBScript does not use the New keyword.

AddRef

Adds another reference to the tally (kept by the component) of how many clients are accessing it. In VB this function is accessed by setting an object variable as an instance of a class:

Set ObjectVariableName = [New] ClassName

VBScript does not use the New keyword.
When the object is instantiated, it's Initialize event fires.

Release

Releases a reference to itself by clients. In VB this function is accessed by setting an object variable back to Nothing:

Set objInstance = Nothing

Objects are destroyed when all object variables that point to it are set to Nothing or when the last variable that refers to the object goes out of scope.
It is good practice to explicitly set the object variables to Nothing when they are no longer needed.
When the object is released, its Terminate event fires.

Binding

The binding of an object determines the speed at which the objects methods can be accessed. There are three types of binding:

Late Bound

Aka weakly typed. This binding is slow. The syntax of this implementation is:

Dim ObjectVariableName As Object

Early DispID Bound

This is medium fast binding. The syntax of this implementation is:

Dim ObjectVariableName As ClassName

Early vtable Bound

Aka strongly typed). This is the fastest binding. This is usually used by languages like C.

Object Methods

You can also set objects of other programs, especially ActiveX component compliant programs, to object variables by using the following functions.

GetObject()

PathName indicates the path and file of the object. If PathName is omitted, then AppName.ObjectTypeOrClass is required to create a new object instance.

Dim obj As Object
Set obj1 = GetObject([PathName] [,AppName.ObjectTypeOrClass])

Set obj = GetObject("", "Excel.Application")
'Just like CreateObject on local machine.

Set obj = GetObject("C:\Foobar\text.xls")
'Points to an object in another directory

CreateObject()

ServerName indicates where the object will be created. If ServerName is omitted, then the local machine will be used.

Dim obj As Object
Set obj = CreateObject(AppName.ObjectTypeOrClass, [ServerName])

Set obj = CreateObject("Excel.Application", "ServerA")
'Make a new object on ServerA

In ASP (Active Server Pages) the built-in Server object can be used to create other objects that are registered on that server.

Set obj = Server.CreateObject("progID")

<% Set objAd = Server.CreateObject("MSWC.AdRotator") %>

<% Set objDog = Server.CreateObject("GHCustom.Dog") %>

Page Modified: (Hand noted: 2007-09-20 21:13:44Z) (Auto noted: 2007-11-17 06:36:57Z)