Computers & Networks

COM clients communicate with objects through COM interfaces. Interfaces are groups of logically or semantically related routines which provide communication between a provider of a service (server object) and its clients

For example, every COM object implements the basic interface,IUNKnown which tells the client what interfaces are available on the COM object.

bjects can have multiple interfaces, where each interface implements a feature. An interface provides a way to convey to the client what service it provides, without providing implementation details of how or where the object provides this service.

Key aspects of COM interfaces are as follows:

Once published, interfaces are immutable; that is, they do not change. You can rely on an interface to provide a specific set of functions. Additional functionality is provided by additional interfaces.
By convention, COM interface identifiers begin with a capital I and a symbolic name that defines the interface, such as IMalloc or IPersist.
Interfaces are guaranteed to have a unique identification, called a Globally Unique Identifier (GUID), which is a 128-bit randomly generated number. Interface GUIDs are called Interface Identifiers (IIDs). This eliminates naming conflicts between different versions of a product or different products.

Interfaces are language independent. You can use any language to implement a COM interface as long as the language supports a structure of pointers, and can call a function through a pointer either explicitly or implicitly.
Interfaces are not objects themselves; they provide a way to access an object. Therefore, clients do not access data directly; clients access data through an interface pointer

Interfaces are always inherited from the fundamental interface,IUNKnown

Interfaces can be redirected by COM through proxies to enable interface method calls to call between threads, processes, and networked machines, all without the client or server objects ever being aware of the redirection

Fundamentals Of IUKnown

All COM objects must support the fundamental interface, called IUnknown, which contains the following routines:

QueryInterface    Provides pointers to other interfaces that the object supports.
AddRef and Release    Simple reference counting methods that keep track of the object’s lifetime so that an object can delete itself when the client no longer needs its service.
Clients obtain pointers to other interfaces through the IUnknown method, QueryInterface. QueryInterface knows about every interface in the server object and can give a client a pointer to the requested interface. When receiving a pointer to an interface, the client is assured that it can call any method of the interface.
Objects track their own lifetime through the IUnknown methods, AddRef and Release, which are simple reference counting methods. As long as an object’s reference count is nonzero, the object remains in memory. Once the reference count reaches zero, the interface implementation can safely dispose of the underlying object(s).

Check Here

Article Source: ArticlesBase.com - COM Interfaces