What Is an API?
API stands for application programming interface. An API is a set of functions you can use to work with a component, application, or operating system. Typically, an API consists of one or more DLLs that provide some specific functionality.
DLLs are files that contain functions that can be called from any application running in Microsoft® Windows®. At run time, a function in a DLL is linked dynamically into an application that calls it. No matter how many applications call a function in a DLL, that function exists only in a single file on the disk, and the DLL is created only when in memory.
The API you probably have heard about most frequently is the Windows API, which includes the DLLs that make up the Windows operating system. Every Windows application interacts with the Windows API directly or indirectly. The Windows API ensures all applications running under Windows will behave in a consistent manner.
Note As the Windows operating system has evolved, several versions of the Windows API have been published. Windows 3.1 uses the Win16 API. The Microsoft® Windows NT®, Windows 95, and Windows 98 platforms use the Microsoft® Win32® API.
There are other published APIs available in addition to the Windows API. For example, the Mail Application Programming Interface (MAPI) is a set of DLLs that can be used to write e-mail applications.
Traditionally, APIs are written for C and C++ programmers who are building Windows applications, but the functions in a DLL can be called by other programming languages, including VBA. Because most DLLs are written and documented primarily for C and C++ programmers, calling a DLL function might differ somewhat from calling a VBA function. To work with an API, you must understand how to pass arguments to a DLL function.
Caution Calling the Windows API and other DLL functions can be hazardous to the health of your application. When you call a DLL function directly from your code, you are bypassing some of the safety mechanisms that VBA usually provides for you. If you make a mistake in defining or calling a DLL function (as all programmers eventually do), you might cause an application error (also referred to as a general protection fault, or GPF) in your application. The best strategy is to save your project before you run your code, and to make sure you understand the principles behind calls to DLL functions.