COM+ Programming with Visual Basic

Chapter 4. In-Process Servers

In the last chapter, you learned about the memory layout of VB COM objects. In particular, you learned how VB allocates memory for objects with multiple interfaces. You also learned how VB enables you to request different portions of this memory using the QueryInterface method of the IUnknown interface (henceforth referred to as QI ). Most important was the idea that all interfaces are created equal. In other words, the memory layout of an interface in any language is basically the same—it is a virtual table pointer (vptr) pointing to a virtual table (vtable). A vtable is nothing more than an array of pointers to the addresses of functions in memory. COM rules state that the first three functions in the vtable of a COM interface must be the methods of IUnknown: QueryInterface, AddRef, and Release. You learned from Chapter 3 that Visual Basic built a little object to manage the IUnknown implementation for the entire object. You also learned some of the COM rules for allocating memory in the last chapter. One rule discussed in the chapter was that memory for COM objects must be allocated by the server code. You also learned that the server provides the definitions of its COM objects for the client through a file called a type library.

In this chapter, you will learn the whole story of activation for ActiveX DLLs. The term activation refers to the process that occurs at the API level from the time the client requests a new server object to the time it ...