Created Unassigned: Reg. Error while Saving Excel using NPOI 2.0Beta [12558]

FileStream file = new FileStream(@"C:\test.xls", FileMode.Open, FileAccess.ReadWrite);

HSSFWorkbook wrkBook = new HSSFWorkbook(file);
ISheet ws = wrkBook.GetSheetAt(1);

IRow r = ws.GetRow(20);
ICell c = r.GetCell(3);

c.SetCellValue("ABC");

wrkBook.Write(file);

file.Close();

Here I am getting the error "Cannot access closed file" at line " wrkBook.Write(file);" .

Can anyone please help me ?