Getting error when i try to save file in my local drive using C#.net code

Question

The below code exporting the data from dataset to excel sheet and it will open as Book1.xls but i want to save in my local drive instead of opening. but i am getting below error when i try to save the file

I am getting error like "System.Runtime.InteropServices.COMException(0x800A03EC):Exception from HRESULT:0x800A03EC at Microsoft.Office.Interop.Excel.WorkbookClasss.SaveAs"  when i am trying to save excel file in my local machine using below code .

 private void GenerateReport(DataSet ds)

        {
            Excel.Application objApp;
            Excel._Workbook objBook;
            Excel.Workbooks objBooks;
            Excel.Sheets objSheets;
            Excel._Worksheet objSheet;  
            try
            {
                objApp = new Excel.Application();
                objBooks = objApp.Workbooks;
                objBook = objBooks.Add(System.Reflection.Missing.Value);
                objSheets = objBook.Worksheets;
                objSheet = (Excel._Worksheet)objSheets.get_Item(1);
                Excel.Range range;           

                DataTable table = ds.Tables[0];
                int ColumnIndex = 0;
                foreach (DataColumn col in table.Columns)
                {
                    ColumnIndex++;
                    switch (col.ColumnName)
                    {
                        case "PRINCIPAL": objSheet.Cells[2, ColumnIndex] = "Principal"; break;
                        case "EVENTGROUP": objSheet.Cells[2, ColumnIndex] = "Eventgroup"; break;
                    }

                }
                objSheet.Cells[1, 1] = "Sentry Month End Report.";
           
                range = objSheet.get_Range("A1", "I2");
                range.Font.Bold = true;
                range.Columns.AutoFit();

                int rowIndex = 1;
                foreach (DataRow row in table.Rows)
                {
                    rowIndex++;
                    ColumnIndex = 0;
                    foreach (DataColumn col in table.Columns)
                    {
                        ColumnIndex++;
                        objSheet.Cells[rowIndex + 1, ColumnIndex] = row[col.ColumnName];
                       

                    }
                }
                objSheet.Columns.AutoFit();
             
                //objApp.Visible = true;
                objBook.SaveAs("C:\\SentryReport.xls", "xls", Missing.Value, Missing.Value, Missing.Value, Missing.Value, Excel.XlSaveAsAccessMode.xlNoChange, Missing.Value, Missing.Value, Missing.Value, Missing.Value, Missing.Value);

            }
            finally
            {
                objApp = null;
                objBook = null;
                objBooks = null;
                objSheets = null;
                objSheet = null;
                GC.Collect();
            }
        }

 

Please any one help me where i making mistake in code

Thanks in Advance..