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 separate from the implementation. The different allocation semantics of languages are accommodated by making objects responsible for their own creation and destruction through reference-counting. Casting between different interfaces of an object is achieved through the QueryInterface() function. 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 Microsoft Windows and Apple's Core Foundation 1.3 and later plug-in API,[1] that in any case implement only a subset of the whole COM interface.[2] For some applications, COM has been replaced at least to some extent by the Microsoft .NET framework, and support for Web Services through the Windows Communication Foundation (WCF). However, COM objects can be used with all .NET languages through .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.
COM programmers build their software using COM-aware components. 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, and several of the scripting languages implemented on the Windows platform. 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).
[edit]Interfaces
All COM components must (at the very least) implement the standard IUnknown interface, and thus all COM interfaces are derived from IUnknown. The IUnknown interface consists of three methods: AddRef() and Release(), which implement reference counting and controls the lifetime of interfaces; and QueryInterface(), which by specifying an IID allows a caller to retrieve references to the different interfaces the component implements. The effect of QueryInterface() is similar to dynamic_cast<> in C++ or casts in Java and C#.
A COM component's interfaces are required to exhibit the reflexive, symmetric, and transitive properties. The reflexive property refers to the ability for the QueryInterface() call on a given interface with the interface's ID to return the same instance of the interface. The symmetric property requires that when interface B is retrieved from interface A via QueryInterface(), interface A is retrievable from interface B as well. The transitive property requires that if interface B is obtainable from interface A and interface C is obtainable from interface B, then interface C should be retrievable from interface A.
An interface consists of a pointer to a virtual function 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. This technique of passing structures of function pointers is very similar to the one used by OLE 1.0 to communicate with its system libraries.
COM specifies many other standard interfaces used to allow inter-component communication. For example, one such interface is IStream, which is exposed by components that have data stream semantics (e.g. a FileStream component used to read or write files). It has the expected Read and Write methods to perform stream reads and writes. Another standard interface is IOleObject, which is exposed by components that expect to be linked or embedded into a container. IOleObject contains methods that allow callers to determine the size of the component's bounding rectangle, whether the component supports operations like 'Open', 'Save' and so on.
A class is COM's language-independent way of defining a class in the object-oriented sense.
A class can be a group of similar objects or a class is simply a representation of a type of object; it should be thought of as a blueprint that describes the object.
A coclass supplies concrete implementation(s) of one or more interfaces. In COM, such concrete implementations can be written in any programming language that supports COM component development, e.g. Delphi, C++, Visual Basic, etc.
One of COM's major contributions to the world of Windows development is the awareness of the concept of separation of interface from implementation. An extension of this fundamental concept is the notion of one interface, multiple implementations. This means that at runtime, an application can choose to instantiate an interface from one of many different concrete implementations.
Type libraries contain metadata that represent COM types. However, these types must first be described using Microsoft Interface Definition Language.
This is the common practice in the development of a COM component, i.e. to start with the definition of types using IDL. An IDL file is what COM provides that allows developers to define object-oriented classes, interfaces, structures, enumerations and other user-defined types in a language independent manner. COM IDL is similar in appearance to C/C++ declarations with the addition of keywords such as "interface" and "library" for defining interfaces and collections of classes, respectively. IDL also requires the use of bracketed attributes before declarations to provide additional information, such as the GUIDs of interfaces and the relationships between pointer parameters and length fields.
The IDL file is compiled by the MIDL compiler into a pair of forms for consumption from various languages. For C/C++, the MIDL compiler generates a compiler-independent header file containing struct definitions to match the vtbls of the declared interfaces and a C file containing declarations of the interface GUIDs. 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 Remote Procedure Calls, thus enabling DCOM.
An IDL file may also be compiled by the MIDL compiler into a type library (.TLB file). The binary metadata contained within the type library is meant to be processed by language compilers and runtime environments (e.g. VB, Delphi, the .NET CLR etc.). The end result of such TLB processing is that language-specific constructs are produced that represent the COM class defined in the .TLB (and ultimately that which was defined in the originating IDL file).
[edit]COM as an object framework
The fundamental principles of COM have their roots in Object-Oriented philosophies. It is a platform for the realization of Object-Oriented Development and Deployment.
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 (versus compile time).
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 :
coclass CSomeObject
{
[default] interface ISomeInterface;
[default, source] dispinterface _IMyObjectEvents;
};
The above code fragment declares a COM class named CSomeObject which must implement an interface named ISomeInterface and which supports (not implements) the event interface _IMyObjectEvents.
Ignoring the event interface bit, this is conceptually equivalent to defining a C++ class like this:
class CSomeObject : public ISomeInterface
{
...
...
...
};
where ISomeInterface is a C++ pure virtual class.
Referring once again to the MyObject COM class: once a coclass definition for it has been formalized in an IDL, and a Type Library compiled from it, the onus is on the individual language compiler to read and appropriately interpret this Type Library and then produce whatever code (in the specific compiler's language) necessary for a developer to implement and ultimately produce the binary executable code which can be deemed by COM to be of coclass MyObject.
Once an implementation of a COM coclass is built and is available in the system, next comes the question of how to instantiate it. In languages like C++, we can use the CoCreateInstance() API in which we specify the CLSID (CLSID_CSomeObject) of the coclass as well as the interface (specified by the IID IID_ISomeInterface) from that coclass that we want to use to interact with that coclass. Calling CoCreateInstance() like this:
CoCreateInstance(CLSID_CSomeObject,
NULL,
CLSCTX_INPROC_SERVER,
IID_ISomeInterface,
(void**)&pISomeInterface);
is conceptually equivalent to the following C++ code:
ISomeInterface* pISomeInterface = new CSomeObject();
In the second case, the COM sub-system is used to obtain a pointer to an object that implements the ISomeInterface interface and coclass CLSID_CSomeObject's particular implementation of this interface is required. In the first case, an instance of a C++ class CSomeObject that implements the interface ISomeInterface is created.
A coclass, then, is an object-oriented class in the COM world. The main feature of the coclass is that it is (1) binary in nature and consequently (2) programming language-independent.
[edit]Registry
Main article: Windows Registry
In Windows, COM classes, interfaces and type libraries are listed by GUIDs in the registry, under HKEY_CLASSES_ROOT\CLSID for classes and HKEY_CLASSES_ROOT\Interface for interfaces. The COM libraries use the registry to locate either the correct local libraries for each COM object or the network location for a remote service.
Under the key HKCR\clsid, the following are specified:
-> Inprocserver32 = object is to be
loaded into a process +
Path to file/object and readable name
HKCR\interface:
example: ISTREAM, IRPCSTUB, IMESSAGEFILTER
connects to a CLSID. You can specify
NUMMETHODS and PROXYSTUB(if web-object)
HKCR\typelib
One or more CLSID can be grouped into type library.
it contains parameters for linking in COM.
The rest of the info in the COM parts of the
REGISTRY, is to give an application/object
a CLSID.
[edit]Reference counting
The most fundamental COM interface of all, IUnknown (from which all COM interfaces must be derived), supports two main concepts: feature exploration through the QueryInterface method, and object lifetime management by including AddRef() and Release(). Reference counts and feature exploration apply to objects (not to each interface on an object) and thus must have a centralized implementation.
The COM specifications require a technique called reference counting to ensure that individual objects remain alive as long as there are clients which have acquired access to one or more of its interfaces and, conversely, that the same object is properly disposed of when all code that used the object have finished with it and no longer require it. A COM object is responsible for freeing its own memory once its reference count drops to zero.
For its implementation, a COM Object usually maintains an integer value that is used for reference counting. When AddRef() is called via any of object's interfaces, this integer value is incremented. When Release() is called, this integer is decremented. AddRef() and Release() are the only means by which a client of a COM object is able to influence its lifetime. The internal integer value remains a private member of the COM object and will never be directly accessible.
The purpose of AddRef() is to indicate to the COM object that an additional reference to itself has been affected and hence it is necessary to remain alive as long as this reference is still valid. Conversely, the purpose of Release() is to indicate to the COM object that a client (or a part of the client's code) has no further need for it and hence if this reference count has dropped to zero, it may be time to destroy itself.
Certain languages (e.g. Visual Basic) provide automatic reference counting so that COM object developers need not explicitly maintain any internal reference counter in their source codes. Using COM in C, explicit reference counting is needed. In C++, a coder may write the reference counting code or use a smart pointer that will manage all the reference counting.
The following is a general guideline calling AddRef() and Release() to facilitate proper reference counting in COM object:
Functions (whether object methods or global functions) that return interface references (via return value or via "out" parameter) should increment the reference count of the underlying object before returning. Hence internally within the function or method, AddRef() is called on the interface reference (to be returned). An example of this is the QueryInterface() method of the IUnknown interface. Hence it is imperative that developers be aware that the returned interface reference has already been reference count incremented and not call AddRef() on the returned interface reference yet another time.
Release() must be called on an interface reference before that interface's pointer is overwritten or goes out of scope.
If a copy is made on an interface reference pointer, AddRef() should be called on that pointer. After all, in this case, we are actually creating another reference on the underlying object.
AddRef() and Release() must be called on the specific interface which is being referenced since an object may implement per-interface reference counts in order to allocate internal resources only for the interfaces which are being referenced.
Extra calls to these functions are not sent out to remote objects over the wire; a proxy keeps only one reference on the remote object and maintains its own local reference count.
To facilitate and promote COM development, Microsoft introduced ATL (Active Template Library) for C++ developers. ATL provides for a higher-level COM development paradigm. It also shields COM client application developers from the need to directly maintain reference counting, by providing smart pointer objects.
Other libraries and languages that are COM-aware include the Microsoft Foundation Classes, the VC Compiler COM Support, VBScript, Visual Basic, ECMAScript (JavaScript) and Borland Delphi.
[edit]Instantiation
COM standardizes the instantiation (i.e. creation) process of COM objects by requiring the use of Class Factories. In order for a COM object to be created, two associated items must exist:
A Class ID.
A Class Factory.
|
Each COM Class or CoClass must be associated with a unique Class ID (a GUID). It must also be associated with its own Class Factory (that is achieved by using a centralized registry). A Class Factory is itself a COM object. It is an object that must expose the IClassFactory or IClassFactory2 (the latter with licensing support) interface. The responsibility of such an object is to create other objects. A class factory object is usually contained within the same executable code (i.e. the server code) as the COM object itself. When a class factory is called upon to create a target object, this target object's class id must be provided. This is how the class factory knows which class of object to instantiate. A single class factory object may create objects of more than one class. That is, two objects of different class ids may be created by the same class factory object. However, this is transparent to the COM system. By delegating the responsibility of object creation into a separate object, a greater level of abstraction is promoted, and the developer is given greater flexibility. For example, implementation of the Singleton and other creation patterns is facilitated. Also, the calling application is shielded from the COM object's memory allocation semantics by the factory object. |
The following is a general outline of the sequence of object creation via its class factory:
The object's class factory is obtained via the CoGetClassObject() API (a standard Windows API).
As part of the call to CoGetClassObject(), the Class ID of the object (to be created) must be supplied. The following C++ code demonstrates this:
IClassFactory* pIClassFactory = NULL;
CoGetClassObject(CLSID_SomeObject,
CLSCTX_ALL,
NULL,
IID_IClassFactory,
(LPVOID*)&pIClassFactory);
The above code indicates that the Class Factory object of a COM object, which is identified by the class id CLSID_SomeObject, is required. This class factory object is returned by way of its IClassFactory interface.
The returned class factory object is then requested to create an instance of the originally intended COM object. The following C++ code demonstrates this:
ISomeObject* pISomeObject = NULL;
if (pIClassFactory)
{
pIClassFactory->CreateInstance (NULL,
IID_ISomeObject,
(LPVOID*)&pISomeObject);
pIClassFactory->Release();
pIClassFactory = NULL;
}
The above code indicates the use of the Class Factory object's CreateInstance() method to create an object which exposes an interface identified by the IID_ISomeObject GUID. A pointer to the ISomeObject interface of this object is returned. Also note that because the class factory object is itself a COM object, it needs to be released when it is no longer required (i.e. its Release() method must be called).
The above demonstrates, at the most basic level, the use of a class factory to instantiate an object. Higher level constructs are also available, some of which do not even involve direct use of the Windows APIs.
For example, the CoCreateInstance() API can be used by an application to directly create a COM object without acquiring the object's class factory. However, internally, the CoCreateInstance() API itself will invoke the CoGetClassObject() API to obtain the object's class factory and then use the class factory's CreateInstance() method to create the COM object.
VBScript supplies the New keyword as well as the CreateObject() global function for object instantiation. These language constructs encapsulate the acquisition of the class factory object of the target object (via the CoGetClassObject() API) followed by the invocation of the IClassFactory::CreateInstance() method.
Other languages, e.g. PowerBuilder's PowerScript may also provide their own high-level object creation constructs. However, CoGetClassObject() and the IClassFactory interface remain the most fundamental object creation technique.
At the time of the inception of COM technologies, the only way for a client to find out what features an object would offer was to actually create one instance and call into its QueryInterface method (part of the required IUnknown interface). This way of exploration became awkward for many applications, including the selection of appropriate components for a certain task, and tools to help a developer understand how to use methods provided by an object.
As a result, COM Type Libraries were introduced, through which components can describe themselves. A type library contains information such as the CLSID of a component, the IIDs of the interfaces the component implements, and descriptions of each of the methods of those interfaces. Type libraries are typically used by Rapid Application Development (RAD) environments such as Visual Basic or Visual Studio to assist developers of client applications.
COM is a binary standard (also said to be language agnostic) and may be developed in any programming language capable of understanding and implementing its binary defined data types and interfaces.
Runtime libraries (in extreme situations, the programmers) are responsible for entering and leaving the COM environment, instantiating and reference counting COM objects, querying objects for version information, coding to take advantage of advanced object versions, and coding graceful degradation of function when newer versions are not available.
COM objects may be instantiated and referenced from within a process, across process boundaries within a computer, and across a network, using the DCOM technology. Out-of-process and remote objects may use marshalling to send method calls and return values back and forth. The marshalling is invisible to the object and the code using the object.
In COM, threading issues are addressed by a concept known as "apartment models". Here the term "apartment" refers to an execution context wherein a single thread or a group of threads is associated with one or more COM objects.
Apartments stipulate the following general guidelines for participating threads and objects:
Each COM object is associated with one and only one apartment. This is decided at the time the object is created at runtime. After this initial setup, the object remains in that apartment throughout its lifetime.
A COM thread (i.e., a thread in which COM objects are created or COM method calls are made) is also associated with an apartment. Like COM objects, the apartment with which a thread is associated is also decided at initialization time. Each COM thread also remains in its designated apartment until it terminates.
Threads and objects which belong to the same apartment are said to follow the same thread access rules. Method calls which are made inside the same apartment are performed directly without any assistance from COM.
Threads and objects from different apartments are said to play by different thread access rules. Method calls made across apartments are achieved via marshalling. This requires the use of proxies and stubs.
There are three types of Apartment Models in the COM world: Single-Threaded Apartment (STA), Multi-Threaded Apartment (MTA), and Neutral Apartment. Each apartment represents one mechanism whereby an object's internal state may be synchronized across multiple threads.
The Single-Threaded Apartment (STA) model is a very commonly used model. Here, a COM object stands in a position similar to a desktop application's user interface. In an STA model, a single thread is dedicated to drive an object's methods, i.e. a single thread is always used to execute the methods of the object. In such an arrangement, method calls from threads outside of the apartment are marshalled and automatically queued by the system (via a standard Windows message queue). Thus, there is no worry about race conditions or lack of synchronicity because each method call of an object is always executed to completion before another is invoked.
If the COM object's methods perform their own synchronization, multiple threads dedicated to calling methods on the COM object are permitted. This is termed the Multiple Threaded Apartment (MTA). Calls to an MTA object from a thread in an STA are also marshaled. A process can consist of multiple COM objects, some of which may use STA and others of which may use MTA. The Thread Neutral Apartment allows different threads, none of which is necessarily dedicated to calling methods on the object, to make such calls. The only provision is that all methods on the object must be serially reentrant.
Since COM has a fairly complex implementation, programmers can be distracted by some of the "plumbing" issues.
When an STA is initialized it creates a hidden window that is used for inter-apartment and inter-process message routing. This window must have its message queue regularly pumped. This construct is known as a message pump. On earlier versions of Windows, failure to do so could cause system-wide deadlocks. This problem is complicated by some Windows APIs that initialize COM as part of their implementation, which causes a leak of implementation details.
Reference counting within COM may cause problems if two or more objects are circularly referenced. The design of an application must take this into account so that objects are not left orphaned.
Objects may also be left with active reference counts if the COM "event sink" model is used. Since the object that fires the event needs a reference to the object reacting to the event, the object's reference count will never reach zero.
Reference cycles are typically broken using either out-of-band termination or split identities. In the out of band termination technique, an object exposes a method which, when called, forces it to drop its references to other objects, thereby breaking the cycle. In the split identity technique, a single implementation exposes two separate COM objects (also known as identities). This creates a weak reference between the COM objects, preventing a reference cycle.
Because COM components are usually implemented in DLL files and registration allows only single version of a DLL they are subject of "DLL hell" effect. Registration-free COM capability eliminates the problem.
RegFree COM (or Registration-Free COM) is a technology introduced with Windows XP that allows Component Object Model (COM) components to store activation metadata and CLSID (Class ID) for the component without using the registry. Instead, the metadata and CLSIDs of the classes implemented in the component are declared in an assembly manifest (described using XML), stored either as a resource in the executable or as a separate file installed with the component. This allows multiple versions of the same component to be installed in different directories, described by their own manifests, as well as XCOPY deployment.[8] This technique has limited support for EXE COM server and cannot be used for system-wide components such as MDAC, MSXML, DirectX or Internet Explorer.
During application loading, the Windows loader searches for the manifest.[10] If it is present, the loader adds information from it to the activation context When the COM class factory tries to instantiate a class, the activation context is first checked to see if an implementation for the CLSID can be found. Only if the lookup fails is the registry scanned
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