DOCUMENT:Q153546 27-JAN-2001 [vbwin] TITLE :HOWTO: Pass Optional Parameters to a C DLL Function from VB PRODUCT :Microsoft Visual Basic for Windows PROD/VER::4.0,5.0,6.0 OPER/SYS: KEYWORDS:kbAPI kbSDKWin32 kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Visual Basic Learning Edition for Windows, versions 6.0, 5.0 - Microsoft Visual Basic Professional Edition for Windows, versions 6.0, 5.0 - Microsoft Visual Basic Enterprise Edition for Windows, versions 6.0, 5.0 - Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 - Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 - Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 - Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 ------------------------------------------------------------------------------- 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 -------------------- 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. Additional query words: ====================================================================== Keywords : kbAPI kbSDKWin32 kbVBp kbVBp400 kbVBp500 kbVBp600 kbGrpDSVB kbDSupport Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbVB400Search kbVB400 kbZNotKeyword3 Version : :4.0,5.0,6.0 Issue type : kbhowto ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.