The architectures of connection points and type information were developed together. In fact, connection points as implemented by high-level languages such as Microsoft Visual Basic and Java could not exist without the presence of type information. As you saw in Chapter 3, a type library is the binary form of an interface definition. The Microsoft Interface Definition Language (MIDL) compiler was initially designed to generate C language bindings for programmers using Remote Procedure Calls1 (RPCs) but has since been extended to create type library files. MIDL does not contain all the code necessary to create a type library, however. Microsoft has defined and implemented interfaces that do this job; MIDL is only a consumer of these interfaces.
The two primary interfaces that MIDL uses to create type libraries are ICreateTypeLib(2) and ICreateTypeInfo(2). Together, these interfaces enable MIDL to generate a type library that describes practically any interface you might write in IDL.2 The ITypeLib(2) and ITypeInfo(2) interfaces implemented by COM+ enable applications to read type information stored in type libraries.
Other programs besides the MIDL compiler use the ICreateTypeLib(2) and ICreateTypeInfo(2) interfaces. Visual Basic, for example, automatically generates a type library for all components. So does Microsoft Visual J++.3 To understand the structure and contents of a type library, you must take at least a cursory look at these interfaces. In the following section, we'll use these interfaces and their associated methods to create a type library that exactly matches the type library produced by the MIDL compiler when it is fed the IDL file shown in Listing 9-1. The numbers in parentheses in the comments correspond to the numbers in the code that creates those elements of type information later in this section.
mylib.idl
import "unknwn.idl";
[ object, uuid(10000001-0000-0000-0000-000000000001), // (6)
oleautomation ] // (7)
interface ISum // (5)
: IUnknown // (12)
{
HRESULT Sum(int x, int y, [out, retval] int* retval);// (13)
}
[ uuid(10000003-0000-0000-0000-000000000001), // (1)
helpstring("Inside COM+ Component Type Library"), // (4)
version(1.0) ] // (3)
library Component // (2)
{
importlib("stdole32.tlb"); // (12)
interface ISum;
[ uuid(10000002-0000-0000-0000-000000000001) ] // (9)
coclass InsideCOM // (8)
{
[default] // (11)
interface ISum; // (10)
}
};
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Listing 9-1. An IDL file containing library information that the MIDL compiler uses to produce a type library file.