Why does Microsoft.Office.Interop.Excel corrupts my excel file?

I found this lib on the net

// A library to handle excel files in a simple way.
// Copyright (C) 2009  Gorka Suárez García
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with this program.  If not, see .
using System;
using System.Collections.Generic;
using System.Reflection;
using Microsoft.Office.Interop.Excel;

namespace Excel
{
    /// <summary>
    /// This class is used to handle an excel file to write and read from it.
    /// Author: Gorka Suárez García
    /// </summary>
    public class ExcelHandler
    {
        /// <summary>
        /// The excel application instance.
        /// </summary>
        private ApplicationClass app;

        /// <summary>
        /// The excel book.
        /// </summary>
        private Workbook book;

        /// <summary>
        /// The path of the excel file.
        /// </summary>
        private string path;

        /// <summary>
        /// Constructs a new ExcelHandler object.
        /// </summary>
        public ExcelHandler()
        {
            this.app = null;
            this.book = null;
            this.path = null;
        }

        /// <summary>
        /// Destroys the ExcelHandler object.
        /// </summary>
        ~ExcelHandler()
        {
            if (this.app != null)
            {
                this.app.Quit();
            }
        }

        /// <summary>
        /// Opens an excel file.
        /// </summary>
        /// <param name="path">The file to open.</param>
        public void Open(string path)
        {
            this.path = path;

            this.app = new ApplicationClass();
            this.app.Visible = true;
            /*this.app.ScreenUpdating = false;
            this.app.DisplayAlerts = false;*/

            this.book = this.app.Workbooks.Open(this.path, Missing.Value, Missing.Value, Missing.Value,
                                                Missing.Value, Missing.Value, Missing.Value, Missing.Value,
                                                Missing.Value, Missing.Value, Missing.Value, Missing.Value,
                                                Missing.Value, Missing.Value, Missing.Value);

            if (this.book == null)
                throw new Exception("Can't open the excel book file.");
        }

        /// <summary>
        /// Writes a value in a cell.
        /// </summary>
        /// <param name="sheet">The sheet to write.</param>
        /// <param name="cell">The cell to write.</param>
        /// <param name="value">The value to write.</param>
        public void Write(string sheet, string cell, string value)
        {
            Worksheet wsheet = this.getSheet(sheet);
            Range range = wsheet.get_Range(cell, cell);
            range.Value2 = value;
        }

        /// <summary>
        /// Reads a value from a cell.
        /// </summary>
        /// <param name="sheet">The sheet to read.</param>
        /// <param name="cell">The cell to read.</param>
        /// <returns>The value from the cell.</returns>
        public string Read(string sheet, string cell)
        {
            Worksheet wsheet = this.getSheet(sheet);
            Range range = wsheet.get_Range(cell, cell);

            if (range.Value2 != null)
                return range.Value2.ToString();
            else
                return "";
        }

        /// <summary>
        /// Clears the content of the excel book.
        /// </summary>
        public void Clear()
        {
            Worksheet sheet = null;
            for (int i = 1; i <= this.book.Worksheets.Count; i++)
            {
                sheet = (Worksheet)this.book.Worksheets[i];
                sheet.Cells.Clear();
            }
        }

        /// <summary>
        /// Closes the excel file.
        /// </summary>
        public void Close()
        {
            this.book.SaveAs(this.path, XlFileFormat.xlWorkbookNormal, Missing.Value, Missing.Value,
                             false, false, XlSaveAsAccessMode.xlShared, false, false, Missing.Value,
                             Missing.Value, Missing.Value);
            this.book.Close(true, Missing.Value, Missing.Value);
            this.app.Quit();

            this.app = null;
            this.book = null;
            this.path = null;
        }

        /// <summary>
        /// Gets all the names of the sheets inside the excel book.
        /// </summary>
        /// <returns>A list of the sheets names.</returns>
        public string[] GetSheetsNames()
        {
            List<string> names = new List<string>();
            Worksheet sheet = null;

            for (int i = 1; i <= this.book.Worksheets.Count; i++)
            {
                sheet = (Worksheet)this.book.Worksheets[i];
                names.Add(sheet.Name);
            }

            return names.ToArray();
        }

        /// <summary>
        /// Gets a sheet we're looking for.
        /// </summary>
        /// <param name="name">The name of the sheet.</param>
        /// <returns>The sheet we're looking for.</returns>
        protected Worksheet getSheet(string name)
        {
            int index = this.getSheetIndex(name);
            if (index == 0)
                throw new Exception("Invalid sheet name.");

            Worksheet sheet = (Worksheet)this.book.Worksheets[index];
            return sheet;
        }

        /// <summary>
        /// Gets the index of a sheet we're looking for.
        /// </summary>
        /// <param name="name">The name of the sheet.</param>
        /// <returns>The index of the sheet we're looking for.</returns>
        protected int getSheetIndex(string name)
        {
            Worksheet sheet = null;
            for (int i = 1; i <= this.book.Worksheets.Count; i++)
            {
                sheet = (Worksheet)this.book.Worksheets[i];
                if (sheet.Name == name) return i;
            }
            return 0;
        }
    }
}

And i'm trying to use it to fill a template i made in excel. Like this for example:

Excel.ExcelHandler handler = new Excel.ExcelHandler();
handler.Open(this.filename);
handler.Write("Informe", "E9", row.Fecha);
handler.Close();

it works okay... no errors... but when i try to open the filled开发者_如何转开发 xlsx, Excel says it's corrupt. Any ideas why??? i'm losing my mind over this, any help would be greatly apreciated.

Regards.

---

Try to change XlFileFormat.xlWorkbookNormal to XlFileFormat.xlWorkbookDefault or xlOpenXMLWorkbook. Looks like file is saved in 'XLS' format but with 'XLSX' extension. Hence the error.