FAQ 34.30 How do type libraries improve performance?

The Automation controller can avoid calling GetIDsOfNames at runtime.

An Automation controller can bind to a dispatch interface in one of two ways. One way is for the Automation controller to get the DISPIDs by calling the IDispatch:: GetIDsOfNames function. This is called late binding because the controller binds to the property or method at runtime. Late binding works when nothing is known about the dispatch interface at compile time.

The other way is for the Automation controller to use ID binding. When using ID binding, the Automation controller extracts type information about the Automation object from its type library and maps the names of dispatch methods and properties to DISPIDs. More specifically, the DISPID of each property or method is fixed and is part of the object's type description. If the object is described in a type library, an Automation controller can read the DISPIDs from the type library and does not have to call GetIDsOfNames at runtime.