INFO: Visual Basic Requirements for Exported DLL Functions

MORE INFORMATION

One of the features of Visual Basic is the ability to use a function stored in a DLL. Visual Basic assumes that the exported DLL functions have the same attributes as a Windows API function. In order to use a function from an exported DLL in a Visual Basic program, Visual Basic requires the following:

Requirements for Calling Functions

• Arguments are passed from right to left: Arguments are passed by value unless you specify a pointer or reference type.

• Passing Arguments by Value or by Reference-By default: Visual Basic passes all arguments by reference; arguments passed by reference contain the 32-bit address containing the argument rather than the value of the argument. However, most functions in an exported DLL expect the value of the argument rather than the 32-bit address. To pass an argument by value, use the ByVal keyword.

• Stack-maintenance is handled by the called function.
• Case sensitivity of the name: Under Win16, function names are not case sensitive. Under Win32, function names are now case-sensitive. If you attempt to call a function using a name that is letter similar but case different from the function stored in an exported DLL, a run-time error will occur because the function name does not exist.
  
  For example, User32.dll has a function name called GetSystemMetrics that returns information about the Windows Environment. Using the following declare statement to call this function:
  

        Declare Function GETSYSTEMMETRICS% Lib "User" ByVal nIndex%)
  
  						

  Results in a run-time error because the function, GETSYSTEMMETRICS is not in User32.dll.
  
  To avoid errors caused by incorrectly typing the function name, assign an alias to the function name. For example, you can assign the alias GetWindowsEnv to the GetSystemMetrics function by using the following declare statement:
  

        Declare Function GetWinEnv Lib "User32" Alias GetSystemMetrics _
                         (ByVal int1 As Integer) As Integer.
  
  						

Notes on Creating Exported DLL in Visual C++

This section provides some information for creating an exported DLL that meets the Visual Basic requirements using Microsoft Visual C++. Other C++ compilers may or may not support the keywords used in this section.

Visual Basic requires that the function receiving the arguments also maintain the stack. The Visual C++ keyword that can perform the correct stack maintenance is _stdcall.

The _stdcall keyword decorates the function name with a preceding underscore and appends '@n' where n is the number of bytes required to contain the function's arguments and the return value. For example, if you create a function called GetWindowSize and use the _stdcall keyword to provide stack maintenance, the function is defined as follows:

   int GetWindowSize (int nIndex)
				

The following is the exported name result:

   _GetWindowSize@8
				

Note the preceding underscore that is added by the _stdcall keyword.

To export the file, you can either use the _declspec(dllexport) keyword or a DEF file EXPORT section. The _declspec(dllexport) keyword exports the function but maintains the name decoration. The following example shows how to implement both the _stdcall and _declspec(dllexport) keyword on a function called DisplayMessage:

   _delcspec(dllexport) long _stdcall DisplayMessage (LPSTR szMessage)
				

A DEF file also exports the function name and removes the name decoration. The following example shows how the EXPORT section of a DEF file is implemented for a function called DisplayMessage:

   EXPORTS
      DisplayMessage