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Implementing a Class Factory

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Before OLE can create an object, it needs access to the object's class factory. The class factory implements the IClassFactory interface. For detailed information about this interface, see the COM Programmer's Reference and Inside OLE, Second Edition, published by Microsoft Press.This topics describes what you must do to expose objects for Automation.

It is important to implement a class factory for objects that may be created explicitly through the OLE function CoCreateInstance, or through the Visual Basic New keyword. For example, an application can expose an Application object for creation, but may have many other programmable objects that can be created or destroyed by referencing a member of the Application object. In this case, only the Application object would need a class factory.

For each class factory, you need to implement the following two member functions of the IClassFactory interface, which provide services for OLE API functions. The prototypes for the member functions reside in the file Ole2.h.

  • CreateInstance — Creates an instance of the object's class.

  • LockServer — Prevents the object's server from shutting down, even if the last instance of the object is released. LockServer can improve the performance of applications that frequently create and release objects.

In general, the CreateInstance method should create a new instance of the object's class. For the Application object, however, the CreateInstance method should return the existing instance of the Application object, which is registered in the running object table (ROT).

The class factory object implements the IClassFactory and IUnknown interfaces. All objects must implement IUnknown, which allows ActiveX clients to determine which interfaces the object supports. A class factory can create instances of a class.

The object implements two interfaces: IUnknown and IMyInterface. The interface IMyInterface is a dual interface, which supports both late binding through IDispatch, and early binding through the VTBL. The dual interface provides two ways to invoke the object's methods and properties. IDispatch includes the member functions GetIDsOfNames, GetTypeInfo, GetTypeInfoCount, and Invoke.

Member1 and Member2 are the members of IMyInterface. These members are available as direct entry points through the object's VTBL. They can also be accessed through IDispatch::Invoke.

You must decide how to handle errors that occur in the exposed objects. If an object supports a dual interface and needs to return detailed, contextual error information, you also need to implement the Automation error interface, IErrorInfo.

In addition to writing code to implement objects, you must create a type library and a registration file. Describe the types of exposed objects in the library section of the .idl file or create an .odl file. Use the MIDL compiler to compile the .odl file. A type library (.tlb) file and a header (.h) file are created. The registration file provides information that the operating system and OLE need to locate objects.