Component object model

This paper is adapted from an article appearing in Dr. Dobbs JournalDecember Dobbs Journal and Microsoft Corporation. All rights reserved. Original version available at Microsoftthis version reformatted for printing. Introduction The Component Object Model COM is a software architecture that allows applications to be built from binary software components.

COM is the underlying architecture that forms the foundation for higher-level software services, like those provided by OLE. OLE services span various aspects of commonly needed system functionality, including compound documents, custom controls, interapplication scripting, data transfer, and other software interactions. Figure 1. These services provide distinctly different functionality to the user.

However they share a fundamental requirement for a mechanism that allows binary software components, derived from any combination of pre-existing customers' components and components from different software vendors, to connect to and communicate with each other in a well-defined manner.

This mechanism is supplied by COM, a software architecture that does the following:. It is important to note that COM is a general architecture for component software. Although Microsoft is applying COM to address specific areas such as controls, compound documents, automation, data transfer, storage and naming, and others, any developer can take advantage of the structure and foundation that COM provides.

How does COM enable interoperability? What makes it such a useful and unifying model? To address these questions, it will be helpful to first define the basic COM design principles and architectural concepts.

In doing so, we will examine the specific problems that COM is meant to solve, and how COM provides solutions for these problems. The most fundamental problem that COM solves is: How can a system be designed so that binary executables from different vendors, written in different parts of the world and at different times, are able to interoperate?

To solve this problem, we must first find answers to these four questions:. Additionally, high performance is a requirement for a component software architecture. Although cross-process and cross-network transparency is a laudable goal, it is critical for the commercial success of a binary component marketplace that components interacting within the same address space be able to use each other's services without any undue "system" overhead.

The Component Object Model defines several fundamental concepts that provide the model's structural underpinnings. These include:.

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For any given platform hardware and operating system combinationCOM defines a standard way to lay out virtual function tables vtables in memory, and a standard way to call functions through the vtables.Component Object Model COM is a simple Microsoft specification method that defines a binary standard for exchanging code between two systems, regardless of the OS or programming language. COM provides access to distributed client object services and is used to share cross-platform binary code and programming languages.

COM interactivity occurs via interfacing with expandable COM software components that do not have an impact on underlying implementation objects. COM uses one intra- and interprocess communication model for developer transparency. Developers increase efficiency by modifying service implementation.

COM machine components also share memory, which increases efficiency and provides superior error handling and debugging.

Clients access COM objects through one or more supported interfaces, which are actual contracts specifying COM object service availability. The COM object interface is not tied to any programming language and is usually written in an interface definition language. Because the COM object can have more than one set of available services, COM objects may expose more than one interface to clients.

Component Object Model (COM)

The primary COM module feature is expandability. COM objects continuously incorporate new functions and present new client interfaces for access to old and new services. Toggle navigation Menu. Home Dictionary Tags Development.

The Component Object Model

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Component Object Model

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component object model

The objects follow a strict interface and allow you to query the objects to determine information. Is this what COM objects are? COM is a mechanism that allows the re-use of objects or rather componentsindependently of the languages used by the programmer who implemented the component and the programmer who uses it, and independently of whether the component was implemented in the client's program or elsewhere on the machine or network.

Broadly speaking, each COM component provides an implementation of one or more interfaces. It defines two important aspects of the COM machinery:. All COM components are registered with the system upon installation. If a programmer wants to use a certain component, he needs to:. The COM machinery is extremely complex. For example, implementing or using COM components in C requires a horrendous amount of work, but higher-level languages like Visual Basic have done a lot to ease the implementation and use of COM components.

The benefits are however very real. The system takes care of marshalling method-call arguments, passing them through threads, processes and network connections as needed so that the client code has the impression of using a normal object. Many fundamental parts of Windows are based on COM. Windows Explorer the file managerfor instance, is basically an empty shell. It defines a bunch of COM Interfaces for navigating and displaying tree hierarchies, and all the code that actually displays "My Computer", the drives, the folders and the files is as a set of COM components that implement those interfaces.

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component object model

Active Oldest Votes.The Microsoft Component Object Model COM is a platform-independent, distributed, object-oriented system for creating binary software components that can interact.

To understand COM and therefore all COM-based technologiesit is crucial to understand that it is not an object-oriented language but a standard. Nor does COM specify how an application should be structured; language, structure, and implementation details are left to the application developer. Rather, COM specifies an object model and programming requirements that enable COM objects also called COM components, or sometimes simply objects to interact with other objects.

These objects can be within a single process, in other processes, and can even be on remote computers. They can be written in different languages, and they may be structurally quite dissimilar, which is why COM is referred to as a binary standard ; a standard that applies after a program has been translated to binary machine code. The only language requirement for COM is that code is generated in a language that can create structures of pointers and, either explicitly or implicitly, call functions through pointers.

In general, a software object is made up of a set of data and the functions that manipulate the data. A COM object is one in which access to an object's data is achieved exclusively through one or more sets of related functions. These function sets are called interfacesand the functions of an interface are called methods.

Further, COM requires that the only way to gain access to the methods of an interface is through a pointer to the interface. Besides specifying the basic binary object standard, COM defines certain basic interfaces that provide functions common to all COM-based technologies, and it provides a small number of functions that all components require. COM also defines how objects work together over a distributed environment and has added security features to help provide system and component integrity.

Skip to main content. Contents Exit focus mode. Yes No. Any additional feedback? Skip Submit. Is this page helpful?COM is a platform-independent, distributed, object-oriented system for creating binary software components that can interact. COM objects can be created with a variety of programming languages.

These objects can be within a single process, in other processes, even on remote computers. For information on which operating systems are required to use a particular interface or function, see the Requirements section of the documentation for the interface or function. Skip to main content. Contents Exit focus mode. Where applicable COM objects can be created with a variety of programming languages. Run-time requirements For information on which operating systems are required to use a particular interface or function, see the Requirements section of the documentation for the interface or function.

COM Language Translations Describes the differences between programming languages and describe how to translate COM object syntax from one language to another.

component object model

Automation Automation enables software packages to expose their unique features to scripting tools and other applications. Structured Storage Structured Storage provides file and data persistence in COM by handling a single file as a structured collection of objects known as storages and streams.

Is this page helpful? Yes No. Any additional feedback? Skip Submit. Controls and Property Pages. COM Language Translations. Describes the differences between programming languages and describe how to translate COM object syntax from one language to another. Automation enables software packages to expose their unique features to scripting tools and other applications. Structured Storage.

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Structured Storage provides file and data persistence in COM by handling a single file as a structured collection of objects known as storages and streams.It is used to enable inter-process communication object creation in a large range of programming languages. The essence of COM is a language-neutral way of implementing objects that can be used in environments different from the one in which they were created, even across machine boundaries. For well-authored components, COM allows reuse of objects with no knowledge of their internal implementation, as it forces component implementers to provide well-defined interfaces that are separated from the implementation.

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The different allocation semantics of languages are accommodated by making objects responsible for their own creation and destruction through reference-counting. Type conversion casting between different interfaces of an object is achieved through the QueryInterface method. The preferred method of "inheritance" within COM is the creation of sub-objects to which method "calls" are delegated.

However, COM objects can be used with all. NET languages through. One of the first methods of interprocess communication in Windows was Dynamic Data Exchange DDE[4] first introduced in[5] that allowed sending and receiving messages in so-called "conversations" between applications.

These provided the foundation of many of the ideas behind COM. It was introduced with Word for Windows and Excel inand was later included with Windows, starting with version 3. An example of a compound document is a spreadsheet embedded in a Word for Windows document: as changes are made to the spreadsheet within Excel, they appear automatically inside the Word document. A VBX is a packaged extension in the form of a dynamic-link library DLL that allows objects to be graphically placed in a form and manipulated by properties and methods.

Inwhen version 3. Text conversations and Windows messages had proved not to be flexible enough to allow sharing application features in a robust and extensible way, so COM was created as a new foundation, and OLE changed to OLE2.

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COM was the major software development platform for Windows and, as such, influenced development of a number of supporting technologies.

It was likewise heavily influenced by earlier technologies. As a cross-language component model, COM relies on an interface definition language, or IDL, to describe the objects and associated functions. DCOM Distributed COM extended the reach of COM from merely supporting a single user with separate applications communicating on the Windows desktop, to activating objects running under different security contexts, and on different machines across the network.

With this were added necessary features for configuring which users have authority to create, activate and call objects, for identifying the calling user, as well as specifying required encryption for security of calls.

In order for Microsoft to provide developers with support for distributed transactionsresource pooling, disconnected applications, event publication and subscription, better memory and processor thread management, as well as to position Windows as an alternative to other enterprise-level operating systems, Microsoft introduced a technology called Microsoft Transaction Server MTS on Windows NT 4.

Instances of a component, if coded properly, could be pooled and reused by new calls to its initializing routine without unloading it from memory. Components could also be distributed called from another machine. NET hides most detail from component creation and therefore eases development.

For example, the System. Transactions namespace in. There is limited support for backward compatibility. A COM object may be used in. NET sides, objects using the other technology appear as native objects. See COM Interop. For instance, it allows objects to be transparently marshalled by value across process or machine boundaries more easily.Zanahary (15) odds 5.

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