c# export to excel file getting error message

Question

I am using Excel 2010 I have the following code that I like to export to a xls file but I am getting the following message:

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

Note that is still opens but not sure why I am getting the message

           ....
            a.Fill(dtRecords);

            Response.ClearContent();
            Response.AddHeader("content-disposition", "attachment;filename=Report.xls");
            Response.ContentType = "application/ms-excel";
            string tab = string.Empty;

            foreach (DataColumn datacol in dtRecords.Columns)
            {
                Response.Write(tab + datacol.ColumnName);
                tab = "\t";
            }
            Response.Write("\n");

            foreach (DataRow dr in dtRecords.Rows)
            {
                tab = "";
                for (int j = 0; j < dtRecords.Columns.Count; j++)
                {
                    Response.Write(tab + Convert.ToString(dr[j]));
                    tab = "\t";
                }

                Response.Write("\n");
            }
            Response.End();

Ben Reich · Accepted Answer · 2013-05-23 19:04:26Z

1

I believe the official mime type for .xls files is application/vnd.ms-excel (source)

answered May 23, 2013 at 19:04

Ben Reich

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Comments

Stipo · Accepted Answer · 2013-05-23 22:22:39Z

1

You are getting this message because you are trying to export character separated value (CSV) stream as XLS file.

CSV is textual format, while XLS is complex binary format with much more features than CSV.

MS Excel complains because it first tried to open the file as XLS but it figured out that file is actually not XLS so it shows warning message, but is still smart enough to figure out that file is CSV and act accordingly.

Try saving the file with extension .csv and with content-type text/csv.

answered May 23, 2013 at 22:22

Stipo

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1 Comment

Adam Porad Over a year ago

Excel 2007 and earlier versions didn't provide this warning when opening a file with a .XLS extenstion that was actually formatted as a comma separated values (CSV) or tab separated values (TSV). The warning started with Excel 2010, so users with older versions of Excel won't get this warning message.

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