External Procedure Declaration Can Cause Crash in MS Excel

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For example, to get the Windows Directory, the GetWindowsDirectory function from the Windows API could be used from VBA. The prototype for GetWindows Directory is:

   UINT GetWindowsDirectory(
      LPTSTR lpBuffer,    // address of buffer for Windows directory
      UINT uSize  // size of directory buffer
      );
				

Following is the VBA code that will cause an Application Error during the calling of GetWindowsDirectory:

   Declare Function GetWindowsDirectory Lib "kernel32.dll" _
      Alias "GetWindowsDirectoryA" _
      (ByVal lpBuffer As String, ByVal uSize As Long)

   Sub ThisCrashes()
      Dim lpBuffer As String
      Dim uSize As Long
      uSize = 100
      lpBuffer = Space(uSize)
      GetWindowsDirectory lpBuffer, uSize
      MsgBox lpBuffer
   End Sub
				

If the following Declare statement is used, specifying the return type, the procedure works correctly:

   Declare Function GetWindowsDirectory Lib "kernel32.dll" _
      Alias "GetWindowsDirectoryA" _
    (ByVal lpBuffer As String, ByVal uSize As Long) As Long