StreamWriter

Hi,
I'm trying to create a save dialog and convert to csv format. When i click cancel button, it prompt error. how do i solve it?
and in the dialog, there's a section call "save as". How do i type in .csv text in there?

below my code:
private void btnCSV_Click(object sender, System.EventArgs e)
{
      string fileName = "";
      string fileNameDel = "";
      string fileNameExt = "";
      CrystalDecisions.CrystalReports.Engine.ReportDocument report = new CrystalDecisions.CrystalReports.Engine.ReportDocument( );
      report = this.crystalReportViewer1.ReportSource as CrystalDecisions.CrystalReports.Engine.ReportDocument;
      if (report != null)
      {
            SaveFileDialog SDialog = new SaveFileDialog();
            SDialog.DefaultExt = "xls";
            if (SDialog.ShowDialog() == DialogResult.OK)
            {                    
                  report.ExportToDisk(ExportFormatType.Excel, SDialog.FileName);
                  fileName = SDialog.FileName;
                  fileNameDel = fileName;
                  fileNameExt = fileName;
            }

            Microsoft.Office.Interop.Excel.Application excelApp = new Microsoft.Office.Interop.Excel.Application();
            excelApp.Visible = false;
            excelApp.DisplayAlerts = false;
            Microsoft.Office.Interop.Excel.Workbook excelWk = excelApp.Workbooks.Open(fileName,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing,
                  Type.Missing, Type.Missing);

            fileName = Path.ChangeExtension(fileName, ".csv");
            excelApp.ActiveWorkbook.SaveAs
                  (fileName, Microsoft.Office.Interop.Excel.XlFileFormat.xlCSVWindows,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlNoChange,
                  Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            excelApp.ActiveWorkbook.Save();
            excelApp.Quit();
            fileNameExt = Path.GetExtension(fileNameExt);

            if(fileNameExt == ".xls")
            {
                  File.Delete(fileNameDel);
            }
      }
}

Error:
An unhandled exception of type 'System.Runtime.InteropServices.COMException' occurred in HARTS_RPT.exe
Additional information: '' could not be found. Check the spelling of the file name, and verify that the file location is correct.
If you are trying to open the file from your list of most recently used files on the File menu, make sure that the file has not been renamed, moved, or deleted.