xBk.Close(false, Type.Missing, Type.Missing);
excel.Quit();
System.Runtime.InteropServices.Marshal.ReleaseComObject(xBk); //释放工作表
System.Runtime.InteropServices.Marshal.ReleaseComObject(xSt);
System.Runtime.InteropServices.Marshal.ReleaseComObject(excel);
xSt = null;
excel = null;
xBk = null;
GC.Collect();