Visual Basic Concepts
Because DLL procedures reside in files that are external to your Visual Basic application, you must specify where the procedures are located and identify the arguments with which they should be called. You provide this information with the Declare statement. Once you have declared a DLL procedure, you can use it in your code just like a native Visual Basic procedure.
Important When you call any DLLs directly from Visual Basic, you lose the built-in safety features of the Visual Basic environment. This means that you increase the risk of system failure while testing or debugging your code. To minimize the risk, you need to pay close attention to how you declare DLL procedures, pass arguments, and specify types. In all cases, save your work frequently. Calling DLLs offers you exceptional power, but it can be less forgiving than other types of programming tasks.
In the following example, we'll show how to call a procedure from the Windows API. The function we'll call, SetWindowText, changes the caption on a form. While in practice, you would always change a caption by using Visual Basic's Caption property, this example offers a simple model of declaring and calling a procedure.
Declaring a DLL Procedure
The first step is to declare the procedure in the Declarations section of a module:
Private Declare Function SetWindowText Lib "user32" _ Alias "SetWindowTextA" (ByVal hwnd As Long, _ ByVal lpString As String) As Long
You can find the exact syntax for a procedure by using the API Viewer application, or by searching the Win32api.txt file. If you place the Declare in a Form or Class module, you must precede it with the Private keyword. You declare a DLL procedure only once per project; you can then call it any number of times.
For More Information For more information on declare statements, see the topic "Declaring a DLL Procedure" later in this chapter.
Calling a DLL Procedure
After the function is declared, you call it just as you would a standard Visual Basic function. Here, the procedure has been attached to the Form Load event:
Private Sub Form_Load() SetWindowText Form1.hWnd, "Welcome to VB" End Sub
When this code is run, the function first uses the hWnd property to identify the window where you want to change the caption (Form1.hWnd), then changes the text of that caption to "Welcome to VB."
Remember that Visual Basic can't verify that you are passing correct values to a DLL procedure. If you pass incorrect values, the procedure may fail, which may cause your Visual Basic application to stop. You'll then have to reload and restart your application. Take care when experimenting with DLL procedures and save your work often.
Note Very few API calls recognize the default Variant data type. Your API calls will be much more robust if you declare variables of specific types and use Option Explicit.