How To Pass Optional Parameters to a C DLL Function from VB (153546)

SUMMARY

It is possible to pass optional parameters to a C DLL function from Visual Basic. Visual Basic supports the Optional Keyword in the Declare statement that tells the compiler to pass a VARIANT of type VT_ERROR if the corresponding parameter is omitted in the function call. Otherwise, if the parameter is not omitted, it is passed according to however it may be declared. In any case, nothing is optional on the C side of things -- the function always accepts the fixed number of parameters for which it has been defined.

This article provides a step-by-step example that demonstrates how to pass optional parameters to a C DLL function.

MORE INFORMATION

Step-by-Step Example

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.

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

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.