HOWTO: Pass a NULL String to a Windows API from Visual Basic

• Microsoft Visual Basic Control Creation, Learning, Professional, Enterprise Editions for Windows, version 5.0
• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic for Windows, version 4.0

SUMMARY

This article describes how to pass NULL as a parameter to any Windows API function that requires a pointer to a string from a Visual Basic 4.0 or 5.0 application.

MORE INFORMATION

Many Windows APIs have pointers to strings (for example, LPSTR or LPCSTR) in their parameter lists. The documentation often indicates that special processing occurs if a NULL is passed (instead of a pointer to a string). With earlier versions of Visual Basic, you could accomplish this by using 0& for the parameter. However, Visual Basic 4.0 and 5.0 include a special constant, vbNullString, that you can use when you need to pass NULL to a Windows API.

For example, here's the declaration for the Win32 function FindWindow that accepts two pointers to strings:

      HWND FindWindow(
          LPCTSTR lpClassName,   // pointer to class name
          LPCTSTR lpWindowName   // pointer to window name
          );

      hRet = FindWindow("MyWindowClass", NULL);

      hRet = FindWindow("MyWindowClass", vbNullString)

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