1. Component Object Model
Requirement the code written in one language should be reused in the applications developed using other language.
Solution is COM
COM  it is a specification or a set of rules provided such that language interop can be achieved.
Note in order to specify a code as a component it is manditory that the class definitions has to inherits IUNKNOWN interface.

2. Methods in IUNKNOWN interface
QueryInterface  it is used to maintain the address of all the member functions defined in the class definitions in a VARRAY table structure.
AddRef  It is used to increment the reference counter value by 1 when ever the application uses the definitions of the form component.
Release  when ever the application which uses the COM component definitions is closed in a normal procedure then Release Method is used to release the reference counter information. Note  if the reference counter value is equal to zero then the resources occupied by the COM component will be released.

3. The processing of the component will be done here
COM Types
Com is of 2 types
In Process COM [.dll] 
Operating System
Com Server
Application
In Process COM
PMA
The processing of the component will be done here

4. IN Process COM
When ever a COM type application uses the definitions of the inprocess COM then a request will be given to the COM server where the component will be identified based on its GUID and processing of the component will take place at the PMA [ Private Memory Area ] of the OS and the request will be given back to the application.
Advantages  The performance of the application will be very fast.
Limitations  Always a IN process component depends on a application the Component cant be used independently.
As the processing of the component will be performed at the PMA of the operating system the resources of the OS will be blocked.

5. OUT Process COM COM Server Application PMA Memory Block
Operating System
COM Server
Application
OUT Process COM .exe
PMA
Memory Block

6. OUT Process COM (.exe)
When ever a application uses the OUT process component then a request will be given to the COM server where COM is identified and the processing of the Component will be performed at a individual memory block and the response will be given to the application with the support of PMA.
Advantages  OUT process components can be used within an application or it can be used independently.
The resources of the O/S is not blocked.
Disadvantages  the application which uses the OUT process components will be slow when compared to the applications which uses the in process components.

7. Problem when COM component is used within the .net applications
Operating System
COM Server
.Net Application
COM Component
A dot net application cant send a request directly to the COM Component or it cant send a request to the COM Server

8. Solution to use COM in dot net applications
The environment in which .net applications will be processed is managed environment.
The environment in which COM components will be processed is unmanaged environment.
UnManaged Env
Managed Env
COMMON LANGUAGE RUNTIME
COM Marshalling
COM Server
Runtime Callable Wrapper
COM Component
Un Managed Code
.Net Application
.Net Interop COM
Managed Code

9. When ever the dot net application uses the definitions of the COM Component a request will be given to the CLR which uses the COM marshalling resource and identifies the application uses the definition of the COM Component and it defines a Runtime Callable Wrapper which sends a request to the Com Server where the Component is identified based on its GUID and the definitions of the COM component will be returned to the RCW and based on that definitions a relevant .net interop COM will be defined which will be used by the applications.

10. Microsoft CDO for NTS 1.2 library [COM Component ]
CDONTS { Collabarative Data objects for new Tech. Service }
Usage  used to send / receive Mails
Properties  From,To,Subject,Cc,Bcc,BodyFormat,MailFormat,Body
See example.

11. COM using dot net COM Server Com Type App UnManaged Env Managed Env
COMMON LANGUAGE RUNTIME
COM Callable Wrapper
IUNKNOWN
Interop Com
IDISPATCH
CLASS DEF
.Net assembly [Class Defn’s]
Com Type App

12. When ever a COM type application uses the definitions of the
When ever a COM type application uses the definitions of the .net assembly then a request will be given to the CLR from the COM Server where the CLR uses its COM collable Wrapper [CCW] resource to identify the .net assembly and to define the relevant interrop COM which will be registered in the COM Server and then the COM Type application uses the definitions of the Dot net assembly as if it is using a Com Component only.

13. Steps to define a .net assembly to be used only by the COM type application
Step 1:Select class library template. The main of the application if used in the .net environment will be considered as a name space and if used from the COM type application it will be considered as a project name.
Step 2: open the solution explorer and select class1.cs file right click on it and click on d