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Using Interfaces in Distributed Applications

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In VCL applications, interfaces are a fundamental element in the COM and SOAP distributed object models. Delphi provides base classes for these technologies that extend the basic interface functionality in TInterfacedObject, which simply implements the IInterface interface methods.

When using COM, classes and interfaces are defined in terms of IUnknown rather than IInterface. There is no semantic difference between IUnknown and IInterface, the use of IUnknown is simply a way to adapt Delphi interfaces to the COM definition. COM classes add functionality for using class factories and class identifiers (CLSIDs). Class factories are responsible for creating class instances via CLSIDs. The CLSIDs are used to register and manipulate COM classes. COM classes that have class factories and class identifiers are called CoClasses. CoClasses take advantage of the versioning capabilities of QueryInterface, so that when a software module is updated QueryInterface can be invoked at run time to query the current capabilities of an object.

New versions of old interfaces, as well as any new interfaces or features of an object, can become immediately available to new clients. At the same time, objects retain complete compatibility with existing client code; no recompilation is necessary because interface implementations are hidden (while the methods and parameters remain constant). In COM applications, developers can change the implementation to improve performance, or for any internal reason, without breaking any client code that relies on that interface. For more information about COM interfaces, see Overview of COM technologies.

When distributing an application using SOAP, interfaces are required to carry their own runtime type information (RTTI). The compiler only adds RTTI to an interface when it is compiled using the {$M+} switch. Such interfaces are called invokable interfaces. The descendant of any invokable interface is also invokable. However, if an invokable interface descends from another interface that is not invokable, client applications can only call the methods defined in the invokable interface and its descendants. Methods inherited from the non-invokable ancestors are not compiled with type information and so can't be called by clients.

The easiest way to define invokable interfaces is to define your interface so that it descends from IInvokable. IInvokable is the same as IInterface, except that it is compiled using the {$M+} switch. For more information about Web Service applications that are distributed using SOAP, and about invokable interfaces, see Using Web Services Index.

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