Problem to refer to file

Hi,
I get

System.IO.IOException was unhandled
  HResult=-2146232800
  Message=Cannot modify a read-only container.
  Source=WindowsBase
  StackTrace:
       at System.IO.Packaging.Package.ThrowIfReadOnly()
       at System.IO.Packaging.Package.CreatePart(Uri partUri, String contentType, CompressionOption compressionOption)
       at DocumentFormat.OpenXml.Packaging.OpenXmlPackage.CreateMetroPart(Uri partUri, String contentType)
       at DocumentFormat.OpenXml.Packaging.OpenXmlPart.CreateInternal(OpenXmlPackage openXmlPackage, OpenXmlPart parent, String contentType, String targetExt)
       at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.InitPart[T](T newPart, String contentType, String id)
       at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.InitPart[T](T newPart, String contentType)
       at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.AddNewPartInternal[T]()
       at DocumentFormat.OpenXml.Packaging.OpenXmlPartContainer.AddNewPart[T]()
       at WindowsFormsApplication2.Form1.InsertWorksheet(WorkbookPart workbookPart) in c:\dp13\General Application\WindowsFormsApplication9_4_0_2\WindowsFormsApplication2\Form1.cs:line 457
       at WindowsFormsApplication2.Form1.InsertText(SpreadsheetDocument spreadSheet, String SheetName0, String text, String CellRowRef0, UInt32 CellColRef0) in c:\dp13\General Application\WindowsFormsApplication9_4_0_2\WindowsFormsApplication2\Form1.cs:line 626
       at WindowsFormsApplication2.Form1.menuItem5_Click(Object sender, EventArgs e) in c:\dp13\General Application\WindowsFormsApplication9_4_0_2\WindowsFormsApplication2\Form1.cs:line 158
       at System.Windows.Forms.MenuItem.OnClick(EventArgs e)
       at System.Windows.Forms.MenuItem.MenuItemData.Execute()
       at System.Windows.Forms.Command.Invoke()
       at System.Windows.Forms.Command.DispatchID(Int32 id)
       at System.Windows.Forms.Control.WmCommand(Message& m)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
       at System.Windows.Forms.Form.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.Run(Form mainForm)
       at WindowsFormsApplication2.Program.Main() in c:\dp13\General Application\WindowsFormsApplication9_4_0_2\WindowsFormsApplication2\Program.cs:line 19
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

            WorksheetPart newWorksheetPart = workbookPart.AddNewPart<WorksheetPart>();

        public static void InsertText(SpreadsheetDocument spreadSheet, string SheetName0, string text, string CellRowRef0, uint CellColRef0)
        {
            //using (SpreadsheetDocument spreadSheet = SpreadsheetDocument.Open(docName, true))
            // Open the document for editing.
            //Using(SpreadsheetDocument spreadSheet = SpreadsheetDocument.Open(docName, true))
            //{
            // Get the SharedStringTablePart. If it does not exist, create a new one.
            SharedStringTablePart shareStringPart;
            if (spreadSheet.WorkbookPart.GetPartsOfType<SharedStringTablePart>().Count() > 0)
            {
                shareStringPart = spreadSheet.WorkbookPart.GetPartsOfType<SharedStringTablePart>().First();
            }
            else
            {
                shareStringPart = spreadSheet.WorkbookPart.AddNewPart<SharedStringTablePart>();
            }

            // Insert the text into the SharedStringTablePart.
            // int index = InsertSharedStringItem(text, shareStringPart);

            // Insert a new worksheet.
            WorksheetPart worksheetPart = InsertWorksheet(spreadSheet.WorkbookPart);

            // Insert cell A1 into the new worksheet.
            Cell cell = InsertCellInWorksheet(CellRowRef0, CellColRef0, worksheetPart);

            // Set the value of cell A1.
            cell.CellValue = new CellValue(text);
            cell.DataType = new EnumValue<CellValues>(CellValues.SharedString);

            // Save the new worksheet.
            worksheetPart.Worksheet.Save();
            //}
        }
        private static WorksheetPart InsertWorksheet(WorkbookPart workbookPart)
        {
            // Add a new worksheet part to the workbook.
            WorksheetPart newWorksheetPart = workbookPart.AddNewPart<WorksheetPart>();
            newWorksheetPart.Worksheet = new DocumentFormat.OpenXml.Spreadsheet.Worksheet(new SheetData());
            newWorksheetPart.Worksheet.Save();

            DocumentFormat.OpenXml.Spreadsheet.Sheets sheets = workbookPart.Workbook.GetFirstChild<DocumentFormat.OpenXml.Spreadsheet.Sheets>();
            string relationshipId = workbookPart.GetIdOfPart(newWorksheetPart);

            // Get a unique ID for the new sheet.
            uint sheetId = 1;
            if (sheets.Elements<Sheet>().Count() > 0)
            {
                sheetId = sheets.Elements<Sheet>().Select(s => s.SheetId.Value).Max() + 1;
            }

            string sheetName = "Sheet" + sheetId;

            // Append the new worksheet and associate it with the workbook.
            Sheet sheet = new Sheet() { Id = relationshipId, SheetId = sheetId, Name = sheetName };
            sheets.Append(sheet);
            workbookPart.Workbook.Save();

            return newWorksheetPart;
        }

Many Thanks & Best Regards, Hua Min