are object model interfaces, such as those discussed
earlier in the section titled "Using Interfaces to Implement Polymorphism."
a VB programmer, however, you typically never need to know about IUnknown or IDispatch
which are provided automatically with every COM component. These interfaces are implemented internally
and automatically by VB for every class object that you create in VB.
The purpose of IUnknown is to keep track of open references to an object
and to provide the caller with the functionality that the object supports. IUnknown
has three methods.
AddRef : This method increments an internal reference counter every time a
client creates a new reference to an object.
Release : This method decrements the internal reference counter every time
a client releases a reference to an object. When the counter reaches zero, the object
can be destroyed.
QueryInterface : This method finds out whether an object supports a particular
interface and, if it does, makes that interface's functions available to the calling application.
The purpose of IDispatch is to provide a standard way to access the members
of a given interface for a particular object.
IDispatch has four methods.
GetTypeInfoCount : Returns a non-zero value if type information is available
for this interface.
GetTypeInfo : Returns type information for the interface.
GetIDsOfNames : Finds the internal ID (known as the dispatch ID) of
a particular property of method.
Invoke : Executes a method or accesses a property.
Vtable binding is a particular type of
early binding that VB typically uses when you declare objects in client applications
using the name of a registered object type.