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 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, Python 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). Interfaces All COM components implement the IUnknown (custom) interface, which exposes methods for reference counting and type conversion (casting). 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 A COM class ("coclass") is a concrete implementation of one or more interfaces, and closely resembles classes in object-oriented programming languages. 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 Type libraries contain metadata to represent COM