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Python Programming On Win32 by Mark Hammond, Andy Robinson

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The IDispatch Interface

The COM architecture works well for languages such as C++, where the methods you need to use are known beforehand (i.e., at compile time). You create an object using the standard COM techniques, then perform a QueryInterface() on the object for a particular interface. Once you have the interface, you can make calls on its methods. This architecture does have some drawbacks, notably:

  • There is support for methods, but no support for properties. In many cases, properties would simplify the object model you are attempting to publish.

  • It doesn’t work as well when using higher-level languages than C++. There may be no compile-time step involved at all. The language in use may not support using the .IDL or .H files necessary to obtain the definition of these interfaces.

COM defines the IDispatch interface specifically to meet the requirements of these higher-level languages. The IDispatch interface allows an object to expose an object model (complete with methods and properties) and allows the user of the object to determine the methods and properties available at runtime. This means the methods or properties you need to call can be determined when you need to call them, rather than requiring them to be predefined. You should note that the object model exposed using IDispatch is quite distinct from the IDispatch interface itself; IDispatch is a COM interface that allows an arbitrary object model to be exposed. In other words, IDispatch is not the object model but ...

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