How To Pass a NULL String to a Windows API from Visual Basic

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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 versions 4.0 and higher 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
          );
				

If NULL is passed as the second parameter, FindWindow locates any window of the specified class name. In a C or C++ program, the call might look like this:

      hRet = FindWindow("MyWindowClass", NULL);
				

If you make the same call from Visual Basic, it looks like this:

      hRet = FindWindow("MyWindowClass", vbNullString)
				

Note that using vbNullString is equivalent to passing NULL as the parameter. This is not the same as passing an empty string ("").