How To Pass Optional Parameters to a C DLL Function from VB

MORE INFORMATION

Step-by-Step Example

1. Create a 32-bit Windows C DLL with the following function:

         long _stdcall OptionalParamCall(LPSTR pStr, VARIANT op1, VARIANT op2)
         {
            if (op1.vt == VT_ERROR && op1.scode == DISP_E_PARAMNOTFOUND)
            MessageBox (NULL, "Optional Param1 is Empty!", "Test DLL", MB_OK);
   
            if (op2.vt == VT_ERROR && op2.scode == DISP_E_PARAMNOTFOUND)
            MessageBox (NULL, "Optional Param2 is Empty!", "Test DLL", MB_OK);
   
            MessageBox (NULL, pStr, "Test DLL", MB_OK);
            return 1;
         }
   						

   Export the function in a .DEF file as follows:

         LIBRARY   TESTDLL
   
         CODE      PRELOAD MOVEABLE DISCARDABLE
         DATA      PRELOAD SINGLE
   
         EXPORTS
         OptionalParamCall     @1
   						

   Name this DLL as Testdll.dll and put the file into the \system (Windows 95/98) or \system32 (Windows NT/2000) directory.
2. Create a new project in Visual Basic, and add the following code to Form1:

      Private Declare Function OptionalParamCall Lib "testdll.dll" _
         (ByVal s As String, Optional ByVal op1, Optional ByVal op2) As Long
   
         Private Sub Form_Click()
            ret& = OptionalParamCall("hello")
            ret& = OptionalParamCall("hello", 7)
            ret& = OptionalParamCall("hello", , "world")
            ret& = OptionalParamCall("hello", 8.2, "Mike")
         End Sub
   					

3. Run the Visual Basic program, and click the form. A series of Msgboxes will appear to tell you which optional parameters are empty and give the value of the string passed as the first parameter.