Visual Basic 6 (VB6)

The Windows API is a rich set of functions for accomplishing common tasks. There are several thousand of these functions, and all are documented on Microsoft's website, MSDN. The key is understanding how to use these Windows API's in a Visaul Basic program. That is what this VB6 tutorial is for.

Declaring API functions

API functions are stored in DLL files ("Dynamic Link Libraries"; Sometimes called libraries or libs). For a VB6 program to use functions from a library, they must first be declared. Declaring a function uses the following syntax:

Declare Function (function name) Lib (library) (arguments)

To declare a function, you must first know what library it is in, and then any arguments it accepts. Happily, these are very easy to discover, often by either going to the link above (MSDN) or googling.

Example

Here, we will use a simple function, called beep to demonstrate how to use API functions. First, we must declare it, the beep function is declared as follows:

Private Declare Function Beep Lib "kernel32" _
   (ByVal dwFreq As Long, ByVal dwDuration As Long) As Long

Once a function is declared, it can be used in your VB6 program as if it were a built-in function of Visual Basic. To use the beep function, you must provide a frequency (in hertz), and a duration (in millisecoinds). The following line would beep for 1 second at 440 hertz:

Beep 440, 1000

Here's an example of something very strange you can do with Beep API + a For Loop:

Option Explicit
   
   Private Declare Function Beep Lib "kernel32" _
      (ByVal dwFreq As Long, ByVal dwDuration As Long) As Long
   
   Private Sub Form_Paint()
      Dim i As Integer
      For i = 1 To 2600
         DoEvents
         Beep i, 3
   
         ' "Techno rain": Beep Rnd() * 20000, 5
         ' "Life as a computer in hollywood": Beep Rnd() * 20000, 15        
          ' "YAY! I beat a game from the 70s!": Beep Rnd() * 2000, 20
         ' "Scary part of a game from the 70s": Beep Rnd() * 2000, 200
      Next i
      DoEvents
      Beep 200, 500
End Sub

Finding documentation on the API functions

There are literally thousands of Windows API functions out there, and guessing the code to declare one is difficult. A list of all functions, their uses, and the code to declare them can be found at the following URL on Microsoft's website:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/winprog/winprog/windows_api_start_page.asp

You can also find API functions from the AllAPI Network, which has a collection of every function and it's use in VB6 as well as VB.NET, along with examples and related functions. This website is also home to a VB6 extension called ApiViewer, which is a massive extension to the outdated program that comes with VB6.

http://www.allapi.net/apilist/apilist.php

This tutorail is released under the GNU Free Documentation License 1.2. The original can be found here.