I think this is one of the very common questions in many UG's and interviews. Thought I would share my knowledge about this topic here.
The data present inside the COM objects are manipulated by means of functions or properties called Interfaces. For example, a Textbox is a COM object and its values is been accessed by an interface called "Text" property. Got it ...
Every object when declared, will internally call one interface called, IUNKnown interface. This is applicable for all COM objects. Don't worry lemme simulate this with an example.
As I told interfaces means it will have some functions or properties this IUNKnown interface also contains some 3 important functions. They are:
A. AddRef: A counter is maintained internally of how many times we are declaring or instancising one object. This AddRef method would increment the counter by one.
B. Release: This is a method which will reduce the counter by one. This will normally get fired when we set the object as nothing.
C. Query Interface: Most important one. This only will determine what are the functions or properties available for the object. i.e., when we type "text1" in VB program we are listed with all the properties and methods of a text box control, correct, this query interface only is responsible for getting all those properties and functions for that text box control.
1. Set mail = createobject("adodb.recordset")
2. mail.open "select * from table",cnproject,1,3,1
4. Set mail = nothing
This is a very simple example of declaring our ADO recordset. Here when we declare the record, i.e., Point 1, the AddRef will get fired, incrementing the counter to one. When we execute the Point 4, the Release function will get fired making the counter to get decremented by one. When we type "mail" as in Point 2 and 3, we are listed with functions namely Open and Close. These methods are identified and isplayed to the user by means of Query interface.
Hope I haven't confused anybody with this explanation :)