GetOpenFileName

What am I missing?

strFilter =

"SolidWorks Files (*.sldprt; *.sldasm; *.slddrw)|*.sldprt;*.sldasm;*.slddrw)"

'strFilename = SW.GetOpenFileName("Select File", "", strFilter, lngFileDispName, strFileconfig, strFileDispName)

strFilename = SW.GetOpenFileName(

"Select File", "", strFilter, 0, "", "")

the above code creates an exception error, why?

A first chance exception of type 'System.NullReferenceException' occurred in SWCustomTools2007.dll

SolidworksApi macros