Most APIs in the DirectX SDK are composed of objects and interfaces based on the COM. The COM is a foundation for an object-based system that focuses on reuse of interfaces, and it is the model at the heart of OLE programming. It is also an interface specification from which any number of interfaces can be built. It is an object model at the operating-system level.
Many DirectX APIs are instantiated as a set of OLE objects. You can consider an object to be a black box that represents the hardware and requires communication with applications through an interface. The commands sent to and from the object through the COM interface are called methods. For example, the IDirectDraw2::GetDisplayMode method is sent through the IDirectDraw2 interface to get the current display mode of the display adapter from the DirectDraw object.
Objects can bind to other objects at run time, and they can use the implementation of interfaces provided by the other object. If you know an object is an OLE object, and if you know which interfaces that object supports, your application (or another object) can determine which services the first object can perform. One of the methods all OLE objects inherit, the QueryInterface method, lets you determine which interfaces an object supports and creates pointers to these interfaces. For more information about this method, see IUnknown Interface.