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Component Object ModelRequirement the code written in one languageshould be reused in the applications developedusing other language.

Solution is COM

COM it is a specification or a set of rules providedsuch that language interop can be achieved.

Note in order to specify a code as a component itis manditory that the class definitions has toinherits IUNKNOWN interface.

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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 valueby 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 thenRelease 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.

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COM TypesCom is of 2 types

In Process COM [.dll]

Application

Com Server

In Process COM

PMAThe processing of the component

will be done here

Operating System

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IN Process COMWhen ever a COM type application uses the definitions ofthe inprocess COM then a request will be given to the COMserver where the component will be identified based on its

GUID and processing of the component will take place at thePMA [ Private Memory Area ] of the OS and the request willbe given back to the application.

Advantages The performance of the application will bevery fast.

Limitations Always a IN process component depends on aapplication the Component cant be used independently.

As the processing of the component will be performed at thePMA of the operating system the resources of the OS will beblocked.

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OUT Process COM

Application

Memory Block

COM Server

OUT Process COM.exe

PMA

Operating System

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OUT Process COM (.exe)When ever a application uses the OUT process componentthen a request will be given to the COM server where COM isidentified and the processing of the Component will be

performed at a individual memory block and the responsewill be given to the application with the support of PMA.

Advantages OUT process components can be used withinan application or it can be used independently.

The resources of the O/S is not blocked.

Disadvantages the application which uses the OUTprocess components will be slow when compared to theapplications which uses the in process components.

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Problem when COM component is used

within the .net applications

.Net Application

COM Server

COM Component

Operating System

A dot net application cant send a request directly to the COM Component or itcant send a request to the COM Server

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Solution to use COM in dot net applicationsThe 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

COM Server

COM Component

.NetApplication

.Net Interop COM

COMMON LANGUAGE RUNTIME

COM Marshalling

Runtime Callable Wrapper

Managed Code

Un Managed Code

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When ever the dot net application uses thedefinitions of the COM Component a request will begiven to the CLR which uses the COM marshalling

resource and identifies the application uses thedefinition of the COM Component and it defines aRuntime Callable Wrapper which sends a request tothe Com Server where the Component is identified

based on its GUID and the definitions of the COMcomponent will be returned to the RCW and basedon that definitions a relevant .net interop COM willbe defined which will be used by the applications.

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Microsoft CDO for NTS1.2 library

[COM Component ]CDONTS { Collabarative Data objects for new Tech.

Service }

Usage

used to send / receive MailsProperties

From,To,Subject,Cc,Bcc,BodyFormat,MailFormat,B

ody

See example.

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COM using dot net

UnManaged Env

Managed Env

COM Server

.Net assembly

[Class Defns]

COMMON LANGUAGE RUNTIME

COM Callable Wrapper

Interop Com

IUNKNOWN

IDISPATCH

CLASS DEF

Com TypeApp

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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 theCLR 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 applicationuses the definitions of the Dot net assembly as if it

is using a Com Component only.

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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 beconsidered as a project name.

Step 2: open the solution explorer and select

class1.cs file right click on it and click on d

Steps to define a .net assembly to be used

only by the COM type application