Tutorial 2: Programming Beyond Visual Basic
Using the Windows API

Understanding DLLs

A DLL (dynamic link library) is compiled code that can be accessed only by other programs at the programming level. You put functions and subroutines into DLLs so that you may share them among many programs running at the same time. DLLs are created in languages such as C(++) and Object Pascal (Delphi). The most often used DLLs in the API for 32-bit Windows operating systems are Kernel32.DLL, User32.DLL, and GDI32.DLL. You also can make DLLs in VB, but they are called ActiveX DLLs. For more information about making ActiveX DLLs in VB, see Chapter 26:"Making Object-Oriented Programs with Visual Basic" from the source text Practical Visual Basic 6.

You can program any type of Visual Basic project in order to access the Windows API if you use the Declare statement in the General Declarations section of a module. For example, you may access GetCursorPos, which locates the onscreen mouse pointer position. It is defined for use within Visual Basic as follows:

Declare Function GetCursorPos Lib "user32" _
   Alias "GetCursorPos" (lpPoint As POINTAPI) As Long

In this syntax:

Declare	is a keyword indicating an external procedure declaration.
Function	is the procedure type.
GetCursorPos	is the name of this particular external procedure.
Lib	is a keyword indicating the DLL in which to locate the function.
"user32"	is the The DLL (must be enclosed in quotation marks, no file extension required, and must be in the path-that is, the program must know where to find the DLL).
Alias	is a keyword that indicates a friendly name by which you can refer to the function.
"GetCursorPos"	is the friendly name.
LpPoint	is an argument variable.
As POINTAPI/As Long	is a keyword indicating argument type.
POINTAPI	is an argument type.
Long	is a function return type