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Application Binary Interface by MicrosoftComponent Object Model ( COM) is a standard for introduced by in 1993. It is used to enable creation in a large range of. COM is the basis for several other Microsoft technologies and frameworks, including, the,. 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 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.
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.COM is an interface technology defined and implemented as standard only on and Apple's 1.3 and later plug-in (API). The latter only implements a subset of the whole COM interface. For some applications, COM has been replaced at least to some extent by the framework, and support for through the (WCF).
However, COM objects can be used with all.NET languages through.NET. Networked DCOM uses binary, while WCF encourages the use of -based messaging. COM is very similar to other interface technologies, such as and, although each has its own strengths and weaknesses. Unlike C, COM provides a stable (ABI) that does not change between compiler releases.
This makes COM interfaces attractive for object-oriented C libraries that are to be used by clients compiled using different compiler versions. Contents.History One of the first methods of in Windows was (DDE), first introduced in 1987, that allowed sending and receiving messages in so-called 'conversations' between applications., who was 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 1988 and On Inheritance: What It Means and How To Use It in 1990. These provided the foundation of many of the ideas behind COM. (OLE), Microsoft's first object-based framework, was built on top of DDE and designed specifically for. It was introduced with and in 1991, and was later included with Windows, starting with version 3.1 in 1992. An example of a compound document is a embedded in a Word for Windows document: as changes are made to the spreadsheet within Excel, they appear automatically inside the Word document.In 1991, Microsoft introduced (VBX) with 1.0. A VBX is a packaged extension in the form of a (DLL) that allows objects to be graphically placed in a form and manipulated.
These were later adapted for use by other languages such as. In 1992, when version was released, Microsoft released OLE 2 with its underlying. The COM (ABI) was the same as the MAPI ABI (released in 1992), and like it was based on and ultimately on the 's. 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 1994 (OCXs) were introduced as the successor to VBX controls. At the same time, Microsoft stated that OLE 2 would just be known as 'OLE', and that OLE was no longer an acronym, but a name for all of the company's component technologies. In early 1996, 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 ', and gradually renamed all OLE technologies to ActiveX, except the compound document technology that was used in. Later that year, was introduced as an answer to.Related technologies 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.and As a cross-language component model, COM relies on an interface definition language, or IDL, to describe the objects and associated functions. The COM IDL is based heavily on the feature-rich DCE/RPC IDL, with object-oriented extensions. Microsoft's own implementation of DCE/RPC, known as MSRPC, is heavily used as the primary inter-process communication mechanism for Windows NT services and internal components, making it an obvious choice of foundation.
DCOM 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.COM+ In order for Microsoft to provide developers with support for, resource 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 (MTS) on Windows NT 4. With Windows 2000, that significant extension to COM was incorporated into the operating system (as opposed to the series of external tools provided by ) and renamed COM+. At the same time, Microsoft de-emphasized 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 to call the MTS software, and not the component directly.
Windows 2000 also revised the Component Services control panel application used to configure COM+ components.An advantage of COM+ was that it could be run in 'component farms'. 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 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 (a technology that provides inter-application asynchronous messaging) with components called Queued Components. COM+ events extend the COM+ programming model to support late-bound (see ) events or method calls between the publisher or subscriber and the event system.NET. Main article: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.NET can leverage COM+ via the System.EnterpriseServices namespace, and several of the services that COM+ provides have been duplicated in recent releases of.NET. For example, the System.Transactions namespace in.NET provides the TransactionScope class, which provides transaction management without resorting to COM+. Ad1981b driver download win 7 home premium.
Similarly, can be replaced by with an transport. (MSMQ is a native COM component, however.) There is limited support for backward compatibility. A COM object may be used in.NET by implementing a (RCW). NET objects that conform to certain interface restrictions may be used in COM objects by calling a COM callable wrapper (CCW). From both the COM and.NET sides, objects using the other technology appear as native objects.
See.(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. Main article:Microsoft's new Windows Runtime (or WinRT, not to be confused with ) programming and application model is essentially a COM-based API, although it relies on an enhanced COM. 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 '.winmd' files, which are encoded in ECMA 335 metadata format, the same format that.NET uses with a few modifications.
This common metadata format allows for significantly less overhead than P/Invoke when WinRT is invoked from.NET applications, and its syntax is much simpler.Security COM and ActiveX components are run as native code on the user's machine, with no sandboxing. There are therefore few restrictions on what the code can do. The prior practice of embedding ActiveX components on web pages with did therefore lead to problems with infections. Microsoft recognized the problem with ActiveX as far back as 1996 when Charles Fitzgerald said, 'We never made the claim up front that ActiveX is intrinsically secure'.
Recent versions of Internet Explorer prompt the user before installing ActiveX controls, enabling the user to disallow installation of controls from sites that the user does not trust. The ActiveX controls are with to guarantee their authenticity.It is also possible to disable ActiveX 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. 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 COM programmers build their software using COM-aware. Different component types are identified by class IDs (CLSIDs), which are (GUIDs).
Each COM component exposes its functionality through one or more. The different interfaces supported by a component are distinguished from each other using interface IDs (IIDs), which are GUIDs too. COM interfaces have in several languages, such as, and several of the scripting languages implemented on the Windows platform. All access to components is done through the of the interfaces. This allows techniques such as inter-process, or even inter-computer programming (the latter using the support of DCOM).Interfaces All COM components implement the ( custom) interface, which exposes methods for and (casting). A custom IUnknown interface consists of a pointer to a 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.
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The in-process invocation overhead is therefore comparable to virtual method calls in. In addition to custom interfaces, COM also supports dispatch interfaces inheriting from.
Dispatch interfaces support for. This allows dispatch interfaces to be natively accessed from a wider range of programming languages than custom interfaces.Classes A COM class ('coclass') is a concrete implementation of one or more interfaces, and closely resembles classes in languages. Classes are created based on their class ID 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 can choose which implementation of an interface to instantiate.Interface Definition Language and type libraries Type libraries contain metadata to represent COM types. These types are described using (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 'interface' and 'library' for defining interfaces and collections of classes. 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. For C/C, the MIDL compiler generates a compiler-independent header file containing struct definitions to match the of the declared interfaces and a C file containing declarations of the interface. C source code for a proxy module can also be generated by the MIDL compiler.
This proxy contains method stubs for converting COM calls into to enable DCOM for out-of-process communication. IDL files can also be compiled by the MIDL compiler into a type library (TLB). TLB files contain binary metadata that can be processed by different language compilers and runtime environments (e.g.
Com Objects Tutorial
VB, Delphi,.NET etc.) to generate language-specific constructs to represent the COM types defined in the TLB. For C, this will convert the TLB back to its IDL representation.COM as an object framework Because COM is a runtime framework, types have to be individually identifiable and specifiable at runtime. To achieve this, globally unique identifiers (GUIDs) are used. Each COM type is designated its own GUID for identification at runtime. In order for information on COM types to be accessible at both compile time and runtime, COM uses type libraries.
It is through the effective use of type libraries that COM achieves its capabilities as a dynamic framework for the interaction of objects.Consider the following example coclass definition in an IDL. ISomeInterface. interfaceptr = NULL; HRESULT hr = CoCreateInstance ( CLSIDSomeClass, NULL, CLSCTXALL, IIDISomeInterface, ( void. ) & interfaceptr );In this example, the COM sub-system is used to obtain a pointer to an object that implements ISomeInterface interface, and coclass CLSIDSomeClass's particular implementation of this interface is required.Reference counting All COM objects utilize to manage object lifetimes. The reference counts are controlled by the clients through the AddRef and Release methods in the mandatory IUnknown interface that all COM objects implement. COM objects are then responsible for freeing their own memory when the reference count drops to zero. Certain languages (e.g.
) provide automatic reference counting so that COM object developers need not explicitly maintain any internal reference counter in their source codes. Retrieved October 5, 2010. Microsoft forum:.com. May 30, 2018.com. October 27, 2017.
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Retrieved April 29, 2016. If a call to the CoGetClassObject function finds the class object that is to be loaded in a DLL, CoGetClassObject uses the DLL's exported DllGetClassObject function.
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Codeguru:References. Retrieved March 7, 2006. (1998). Essential COM. Addison-Wesley. Chappell, David (1996).
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