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Component Object Model - Wikipedia. Component Object Model (COM) is a binary- interface standard for software components introduced by Microsoft in 1. It is used to enable inter- process communicationobject creation in a large range of programming languages. COM is the basis for several other Microsoft technologies and frameworks, including OLE, OLE Automation, Browser Helper Object, Active. X, COM+, DCOM, the Windows shell, Direct. X, UMDF and Windows Runtime.
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. 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 Query.
Interface method. The preferred method of . However, COM objects can be used with all . NET languages through .
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- Draw a sphere. The sphere can be represented graphically, or in ASCII art, depending on the language capabilities. Either static or rotational projection is.
- Component Object Model (COM) is a binary-interface standard for software components introduced by Microsoft in 1993. It is used to enable inter-process communication.
- Component Object Model (COM) es una plataforma de Microsoft para componentes de software, introducida en 1993. Esta plataforma es utilizada para permitir la.
- In DirectShow one makes multimedia apps by building graphs where nodes (called filters) process the data (capture or read, convert, compress, write etc.) and graph's.
NET COM Interop. Networked DCOM uses binary proprietary formats, while WCF encourages the use of XML- based SOAP messaging. COM is very similar to other component software interface technologies, such as CORBA and Java Beans, although each has its own strengths and weaknesses. Unlike C++, COM provides a stable application binary interface (ABI) that does not change between compiler releases. Antony Williams involved in the creation of the COM architecture, later distributed two internal papers in Microsoft that embraced the concept of software components: Object Architecture: Dealing With the Unknown – or – Type Safety in a Dynamically Extensible Class Library in 1.
On Inheritance: What It Means and How To Use It in 1. These provided the foundation of many of the ideas behind COM. Object Linking and Embedding (OLE), Microsoft's first object- based framework, was built on top of DDE and designed specifically for compound documents. It was introduced with Word for Windows and Excel in 1. 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. In 1. 99. 1, Microsoft introduced Visual Basic Extensions (VBX) with Visual Basic 1.
A step-by-step guide to creating COM components using Visual C++ - without the help of ATL or MFC. TComInterface
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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. These were later adapted for use by other languages such as Visual C++. In 1. 99. 2, when version 3.
Windows was released, Microsoft released OLE 2 with its underlying object model. The COM Application binary interface (ABI) was the same as the MAPI ABI, which was released in 1. While OLE 1 was focused on compound documents, COM and OLE 2 were designed to address software components in general.
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. In 1. 99. 4 OLE custom controls (OCXs) were introduced as the successor to VBX controls. At the same time, Microsoft stated that OLE 2 would just be known as . In early 1. 99. 6, Microsoft found a new use for OLE Custom Controls, expanding their Web browser's capability to present content, renamed some parts of OLE relating to the Internet .
Later that year, DCOM was introduced as an answer to CORBA. Related technologies. With Windows 2. 00.
COM was incorporated into the operating system (as opposed to the series of external tools provided by MTS) and renamed COM+. At the same time, Microsoft de- emphasized DCOM as a separate entity. Components that made use of COM+ services were handled more directly by the added layer of COM+, in particular by operating system support for interception.
In the first release of MTS, interception was tacked on - installing an MTS component would modify the Windows Registry to call the MTS software, and not the component directly. Windows 2. 00. 0 also revised the Component Services control panel application used to configure COM+ components. An advantage of COM+ was that it could be run in . 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). COM+ and Microsoft Visual Studio provided tools to make it easy to generate client- side proxies, so although DCOM was used to make the remote call, it was easy to do for developers.
COM+ also introduced a subscriber/publisher event mechanism called COM+ Events, and provided a new way of leveraging MSMQ (inter- application asynchronous messaging) with components called Queued Components. COM+ events extend the COM+ programming model to support late- bound (see Late binding) events or method calls between the publisher or subscriber and the event system.
Microsoft . NET provides means both to provide component technology, and to interact with COM+ (via COM- interop- assemblies); . NET provides wrappers to most of the commonly used COM controls. Microsoft . NET hides most detail from component creation and therefore eases development. For example, the System. Transactions namespace in .
NET provides the Transaction. Scope class, which provides transaction management without resorting to COM+.
Similarly, queued components can be replaced by Windows Communication Foundation with an MSMQ transport. A COM object may be used in .
NET by implementing a Runtime Callable Wrapper (RCW). See COM Interop. WCF (Windows Communication Foundation) eases a number of COM's remote execution challenges. For instance, it allows objects to be transparently marshalled by value across process or machine boundaries more easily. Windows Runtime. Because of its COM- like basis, Windows Runtime allows relatively easy interfacing from multiple languages, just as COM does, but it is essentially an unmanaged, native API. The API definitions are, however, stored in .
This common metadata format allows for significantly less overhead than P/Invoke when Win. RT is invoked from . NET applications, and its syntax is much simpler. Security. There are therefore few restrictions on what the code can do. The prior practice of embedding Active.
X components on web pages with Internet Explorer did therefore lead to problems with malware infections. Microsoft recognized the problem with Active.
X as far back as 1. Charles Fitzgerald said, .
The Active. X controls are signed with digital signatures to guarantee their authenticity. It is also possible to disable Active. X controls altogether, or to allow only a selected few.
The transparent support for out- of- process COM servers still promotes software safety in terms of process isolation. This can be useful for decoupling subsystems of large application into separate processes. Process isolation limits state corruption in one process from negatively affecting the integrity of the other processes, since they only communicate through strictly defined interfaces. Thus, only the affected subsystem needs to be restarted in order to regain valid state. This is not the case for subsystems within the same process, where a rogue pointer in one subsystem can randomly corrupt other subsystems. Technical details.
Different component types are identified by class IDs (CLSIDs), which are Globally Unique Identifiers (GUIDs). Each COM component exposes its functionality through one or more interfaces. The different interfaces supported by a component are distinguished from each other using interface IDs (IIDs), which are GUIDs too. COM interfaces have bindings in several languages, such as C, C++, Visual Basic, Delphi, Python. All access to components is done through the methods of the interfaces. This allows techniques such as inter- process, or even inter- computer programming (the latter using the support of DCOM). Interfaces. A custom IUnknown interface consists of a pointer to a virtual method table that contains a list of pointers to the functions that implement the functions declared in the interface, in the same order that they are declared in the interface.
The in- process invocation overhead is therefore comparable to virtual method calls in C++. In addition to custom interfaces, COM also supports dispatch interfaces inheriting from IDispatch. Dispatch interfaces support late binding for OLE Automation. This allows dispatch interfaces to be natively accessed from a wider range of programming languages than custom interfaces.
Classes. Classes are created based on their class ID (CLSID) or based on their programmatic identifier string (progid). Like many object- oriented languages, COM provides a separation of interface from implementation. This distinction is especially strong in COM, where objects cannot be accessed directly, but only through their interfaces. COM also has support for multiple implementations of the same interface, so that clients at runtime can choose which implementation of an interface to instantiate. Interface Definition Language and type libraries. These types are described using Microsoft Interface Definition Language (MSIDL/IDL). IDL files define object- oriented classes, interfaces, structures, enumerations and other user- defined types in a language independent manner.
IDL is similar in appearance to C++ declarations with some additional keywords such as . IDL also supports the use of bracketed attributes before declarations to provide additional information, such as interface GUIDs and the relationships between pointer parameters and length fields. IDL files are compiled by the MIDL compiler.