com ppt final

8/4/2019 COM Ppt Final

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COM


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WHAT IS COM

COM stands for Component Object Model. It is aMicrosoft specification that describes how to createreusable objects for programmers working in aWin32 programming environment. This makes it

easy to develop objects that can be distributedindependently of an application program file.

In earlier versions of VB or other windowsprogramming environment, the COM under other

names such as OLE Server or Activex Server.


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COMPONENT AND OBJECT

It is an independent piece of code that may beshared with several different programs.

Object is a set of code that is designed to bereusable with a well defined interface.

COM is based on Client-Server Model ,each COM

object operates s a Server that receives andprocesses requests from a client program andgenerates responses


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Requests

CLIENT PROGRAMCONTAINER

COM Object(Control)

Response


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COM OBJECTCANINTERACT WITH CLIENT

PROGRAM

IN

THREE

WAYS

:

PROPERTY:

A Property represents variable in your objects,

although its often implemented as a pair ofroutines: One to retrieve the value form the propertyand another to assign a value to the property.


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METHOD AND EVENT

Method is merely a function or subroutine that canbe called from a client program.

An event is a subroutine whose parameters aredefined in the control and that exists in the clientprogram. It is called by the control to informprograms of various situations.


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NECESSICITY OF COM

Components and Objects are used to develop anapplication. Since many languages have the abilityto include objects in source code, why should youuse COM?

COM is the foundation of most things that Microsoftdoes. Without COM ,Visual Basic wouldnt work,

nor would Windows, SQL Server, or any numberof Microsoft products. The true test of Microsofts

confidence in COM is that its applications arebased on COM technology. Word 2000 and Excel2000 are based heavily on COM, as are many otherMicrosoft applications.


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When you build multiple program using objects, theobjects are compiled into the object code. This hasa couple of disadvantages:

If you have multiple programs, each executableprogram has its own copy of the objects. Thismeans that any time you change the objects, youmust recompile all of the programs that use them.

You must use the same programming language that

the objects were written in to create yourapplication programs. This restricts your choice ofprogramming languages


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Since COM Objects are stored in their ownindependent object file, you can change the objectwithout recompiling the applications that use it. Youcan also add new interface to the COM objects to

offer new features and capabilities.


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FROM DDE TO COM

Many of the features we see in todays COM

technology go back many years. COM draws itsroots from a number of different technologyincluding DDE (Dynamic Data Exchange), Object

Linking Embedding.(OLE).


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DDE:

In early days of Windows , people used a clipboardmuch like they do today. Users could copy parts ofa document in program on to the clipboard andpaste them into a document in another program.

However, when dealing with complex graphicaldocuments, users found that a simple clipboarddidnt work very well.

In late 1980s Microsoft developed a technology

called DDE to allow programmers to simplify the

process of sharing the data using a clipboard. Thistechnology was available for only Microsoftapplications, so it was not universal solution.


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OLE:

In 1991 Microsoft created a more general solutionto this problem called OLE . This technologyallowed users to combine documents from multipleapplications into a single compound document

developers could build the best possible wordprocessor without worrying about adding a limitedfunction spread sheet. For example, we can embedexcel worksheet into word document. The word

document is acting as a container for excelworksheet. This is really the foundation of OLE.


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While working on OLE the Microsoft engineersrecognized that the issue of creating compounddocuments was really a special case of determining howtwo or more system or application componentscommunicate with each other. In the past a number of

different techniques were employed. Some times simplesubroutine calls were used. Other times more complexmessaging systems was used. Programmers were oftenrequired to use several different techniques forcommunication among components.

While implementing OLE Microsoft employed a numberof new technologies; for most among them was COM.


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COM:

COM creates a layer of abstraction betweensoftware components . Each COM object isdesigned to operate as a server to a clientapplication or another software module. The COMobject is an instance of a specific class that definesone or more interfaces to the functions it provides.Each interface contains one or more methods toperform specific functions.

With COM, doesnt matter if the component is

implemented as a simple set of subroutines as anindependent process that communicates with theapplication using complex synchronizationtechniques.


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Although COM determines how two components

communicate with one another, OLE requires anumber of other standard methods in addition tothose required by COM. These includes thecapability for the object to present its own user

interface, send events to the controllers containersand lets a container sets the properties of thecontrol.


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SERVICES PROVIDED BY COM

The Services provided by COM are summarizedbelow:

OBJECT MANAGEMENT:

COM provides object management a service byusing reference counting. With reference counting,

COM enables developers to control when objectreferences and their objects are released frommemory.


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OBJECT PERSISTANCE:

COM provides object persistence services that

allow objects to be stored in a file. This isparticularly important when you wish to extract anapplication specific object from a document file.


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STRUCTURED STORAGE:

When objects are stored in files, they are stored

using the structured storage services provided inCOM. With structured storage we can store data ina hierarchical format within a file very much as filesare stored in directories and sub directories.Structured storage allows applications to read and

save items to files with out restricting the lay out ofthe file .


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DATA TRANSFER:

COM provides DATA TRANSFER, a service that

enable you to transfer and share data betweenapplications.

NAMING AND BINDING SERVICES:

With Naming and Binding services, applicationcreate, store and manage objects that providecomplex operations such as downloading of files.These services are usually not directly accessed bythe programmers however you will use theseservices via objects and functions that a classprovides


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WHATS INSIDE A COM OBJECT

COM objects communicate to the world through aseries of interfaces. Standard interfaces aregenerally used to access information about theCOM objects. These interfaces include or perform

various COM related tasks. Custom interfaces arethose interfaces designed by the user as COMobjects.


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The interfaces implemented in one of two basicways:

IN-PROCESS OBJECT SERVERS:

It run in the same address space as the clientprocess. Using them dose not add much moreoverhead to your application than calling a functionor subroutine.It is loaded from a regular DLL file,which contain a set of standardized function and

tables that you load and use COM objectdynamically.


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FINDING AND LOADING IN-PROCESS COM

OBJECTS:

When an object in the COM library is accessed forthe first time, the COM library is loaded into your

address space, just like any other DLL. However,unlike DLL files, which require Windows to searchin a number of places to find the files, COM objectsare quickly located by using a GUID.


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A GUID(Globally Unique Identifier) is a 128 bitnumber i.e. unique, not just in a specific computer,but in the entire world. The same people thatdeveloped the Distributed Computing Environment

derive this value from complex algorithm desined.The GUID generator uses your system clock andthe MAC address from your network interface cardto create a value that is guaranteed to be unique on

your system. If your computer does not have anetwork interface card, the GUID will be createdfrom a value based on your hard disk space andthe total time since the computer was booted.


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OUT OF PROCESS OBJECT SERVERS:

It run in a different address space. They come intwo different flavors: Local and remote. Using theseobjects is more expensive in terms of overhead;however, in long run, they may actually help yourapplication to run faster. While your program callsthem like a function or a subroutine, a lot of workneeds to be done to marshal your request to

another process, wait for results, and then marshalsthe results in the same fashion as an IN-PROCESSCOM object.


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FINDING AND LOADING OUTOF- PROCESS

COM OBJECTS:

Finding out-of-process objects is similar to locatinginprocess objects. The calling program supplies theobjects CLSID, which is used to find information

about the object in the windows registry.


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RUNNING OUT OF PROCESS COM OBJECTS

REMOTELY:

If your computer is attached to a network, windowsincludes the capability to run your object on anothermachine. The same interface that your programsees with a local out-of-process COM object still

exist , but now they direct your request to anothercomputer where the objects exist.

To direct your COM calls to another computer, COMuses a proxy component that intercepts your calls

to methods and properties and passes them on tothe remote computer using Remote ProcedureCalls (RPC).


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INTERFACES

An interface is a contact between a COM serverand a COM client. It specifies the methods,properties, and events that are used tocommunicate between the client and the server.

This is perhaps the most important part of the COMobject, because the interface isolates the internalworking of the COM object from how it is used bythe client.

Every COM object presents at least two interfaces

to the outside world. In most cases, more than twowill be present because a number of interfaces arededicated by the COM architecture.

CC SS G O O O G


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ACCESSING INFORMATION THROUGH AN

INTERFACE:

An interface is a binary specification that allows youto locate a particular variable or function in the codemodule. This information stored in data structurecalled a virtual method table. A VMT is an array of

function pointers, where each pointer points to aparticular method or property contained in theobject.

Each function pointed to by the VTM contains

not only the address of the function, but additionalinformation, such as the name of the function, thedata type it will return, and the name and data typefor each of its parameters.


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BINDING TO AN INTERFACE:

In order to use a COM object, you must find andload the library containing the object. Then youmust create a link between the object variable youplan to use and the desired interface. This process

is known as binding. There are two basic types of binding: early and

late.

In late binding, visual basic is unable to determinethe type of the object your are using.

With early binding, you must declare your objectvariable with a specific object type with thisinformation visual basic is able to bind your variableto the object at designed time.


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COM Object

COM Proxy

Applicationprogram

COM Proxy


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THANK YOU

com ppt final

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