excel file extension problem

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C#3.0

C#

Hello...

I have a windows app where i have to save the data entered by the user in excel sheet via savefiledialog tool...but when i try to save it,it gets saved properly...but when i go to the location where i saved it,n open the file,it opens only once,next time i try to open it,error is displayed that file format or extension is incorrect...n so it cant open...
plz help...

here is my code..

Excell.Application xlApp;
               Excell.Workbook xlWorkBook;
               Excell.Worksheet xlWorkSheet;
               Excell.Range range;
               string str = "";
               int rCnt = 0;
               int cCnt = 0;
               xlApp = new Excell.ApplicationClass();
               object BlankValue = System.Reflection.Missing.Value;
               string docpath = Environment.GetFolderPath(Environment.SpecialFolder
                                                                      .MyDocuments);
               string path1 = string.Format(docpath + @"\Report{0:yyyy_MM_dd}.xlsx",
                                                                 DateTime.Now.Date);
               xlWorkBook = xlApp.Workbooks.Open(path1, 0, true, 1, "", "", true,
                             Microsoft.Office.Interop.Excel.XlPlatform.xlWindows,
                                              "\t", false, false, 0, true, 1, 0);
               xlWorkSheet = (Excell.Worksheet)xlWorkBook.Worksheets.get_Item(1);
               range = xlWorkSheet.UsedRange;
               List<string> Data = new List<string>();
               for (rCnt = 1; rCnt <= range.Rows.Count; rCnt++)
               {
                   for (cCnt = 1; cCnt <= range.Columns.Count; cCnt++)
                   {
                       str = (string)(range.Cells[rCnt, cCnt] as Excell.Range).Value2;
                       Data.Add(str);
                   }
               }
               Stream myStream;
               SaveFileDialog saveFileDialog1 = new SaveFileDialog();
               string path2 = saveFileDialog1.FileName;
               saveFileDialog1.Filter = "Excel Workbook(*.xlsx)|*.xlsx";
               Excell.Application App;
               Excell.Workbook WorkBook;
               Excell.Worksheet WorkSheet;
               App = new Excell.ApplicationClass();
               WorkBook = App.Workbooks.Add(BlankValue);
               WorkSheet = (Excell.Worksheet)WorkBook.Worksheets.get_Item(1);
               //Inserting Data In Sheet1 With Column Headings.
               int count = 0;
               saveFileDialog1.FilterIndex = 1;
               saveFileDialog1.RestoreDirectory = true;
               if (saveFileDialog1.ShowDialog() == DialogResult.OK)
               {
                   if ((myStream = saveFileDialog1.OpenFile()) != null)
                   {
                       StreamWriter wText = new StreamWriter(myStream);
                       for (rCnt = 1; rCnt <= range.Rows.Count; rCnt++)
                       {
                           for (cCnt = 1; cCnt <= range.Columns.Count; cCnt++)
                           {
                               WorkSheet.Cells[rCnt, cCnt] = Data[count];
                               count++;
                           }
                       }
                       //File.Copy(path1, saveFileDialog1.FileName);
                       WorkBook.SaveAs(myStream.ToString()[0], Excell.XlFileFormat
                       .xlWorkbookDefault, BlankValue, BlankValue, false, BlankValue,
                       Excell.XlSaveAsAccessMode.xlNoChange, BlankValue, BlankValue,
                                                 BlankValue, BlankValue, BlankValue);
                       WorkBook.Close(true, null, null);
                       //wText.Close();
                       //myStream.Close();
                       myStream.Dispose();
                       wText.Dispose();
                       App.Quit();
                   }
               }
           }