How do I save an Excel file without getting a format warning when opening, using C# and Excel 2010

Question

I'm using Excel 2010. I'm trying so save my excel file, with this code. It does save a .xls file, but when I open the file I get this message:

The file you are trying to open, 'tre.xls', is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a thrusted source before opening the file. Do you want to open the file now?

If I press yesthe file opens. But what do I need to get rid of this format-popup?

My code:

using System;
using System.Windows.Forms;
using ExcelAddIn1.Classes;
using ExcelAddIn1.Classes.Config;
using Microsoft.Office.Interop.Excel;
using Application = Microsoft.Office.Interop.Excel.Application;
using System.Runtime.InteropServices;


private void Foo(object sender, EventArgs e)
{
    SaveFileDialog saveFileDialog = new SaveFileDialog();
    saveFileDialog.Filter = "Execl files (*.xls)|*.xls";
    saveFileDialog.FilterIndex = 0;
    saveFileDialog.RestoreDirectory = true;
    saveFileDialog.CreatePrompt = true;
    saveFileDialog.FileName = null;
    saveFileDialog.Title = "Save path of the file to be exported";

    if (saveFileDialog.ShowDialog() == DialogResult.OK)
    {
        // Save. 
        // The selected path can be got with saveFileDialog.FileName.ToString()
        Application excelObj = 
            (Application)Marshal.GetActiveObject("Excel.Application");
        Workbook wbook = excelObj.ActiveWorkbook;
        wbook.SaveAs(saveFileDialog.FileName, XlFileFormat.xlWorkbookDefault, 
            Type.Missing, Type.Missing, false, false, 
            XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, 
            Type.Missing, Type.Missing, Type.Missing);
        wbook.Close();
    }
}

When I save in excel in the normal way I get "Book1.xlsx", that have no problem to open.

============= FINAL SOLUTION ============

private void Foo(object sender, EventArgs e)
{
    SaveFileDialog saveFileDialog = new SaveFileDialog();
    saveFileDialog.Filter = "Execl files (*.xls)|*.xls";

    saveFileDialog.FilterIndex = 0;
    saveFileDialog.RestoreDirectory = true;
    saveFileDialog.CreatePrompt = true;
    saveFileDialog.FileName = null;
    saveFileDialog.Title = "Save path of the file to be exported";

    if (saveFileDialog.ShowDialog() == DialogResult.OK)
    {
        Application excelObj = (Application)Marshal.GetActiveObject("Excel.Application");
        Activate(); // <-- added (recommend by msdn, but maybe not nessary here)
        Workbook wbook = excelObj.ActiveWorkbook;
        wbook.SaveAs(saveFileDialog.FileName, XlFileFormat.xlExcel8, Type.Missing,
            Type.Missing, false, false, XlSaveAsAccessMode.xlNoChange, Type.Missing,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing);
        //                wbook.Close(); // <-- don't want this anymore
    }
}

Answer 1

With your code you save the workbook as Open XML because XlFileFormat.xlWorkbookDefault evaluates to XlFileFormat.xlOpenXMLWorkbook for Excel 2007+. This is the reason of the warning: you named the file as XLS but actually it is a XLSX.

You can change the file extension to xlsx in your saveFileDialog.Filter property or force the Excel object to save in the XLS format using XlFileFormat.xlExcel8.

EDIT
Use this to save your document:

wbook.SaveAs(saveFileDialog.FileName, XlFileFormat.xlExcel8, 
            Type.Missing, Type.Missing, false, false, 
            XlSaveAsAccessMode.xlNoChange, Type.Missing, Type.Missing, 
            Type.Missing, Type.Missing, Type.Missing);