the file you are trying to open is in a different format than specified by the file extension in Asp.Net

The Problem (Q-score 14, ranked #44th of 303 in the Excel VBA archive)

The scenario as originally posted in 2013

the file you are trying to open is in a different format than specified by the file extension c# error when trying to open file in excel.

Here is my code

public ActionResult Export(string filterBy)
{
    MemoryStream output = new MemoryStream();
    StreamWriter writer = new StreamWriter(output, Encoding.UTF8);

    var data = City.GetAll().Select(o => new
    {
        CountryName = o.CountryName,
        StateName = o.StateName,
        o.City.Name,
        Title = o.City.STDCode
    }).ToList();
    var grid = new GridView { DataSource = data };
    grid.DataBind();
    var htw = new HtmlTextWriter(writer);

    grid.RenderControl(htw);

    writer.Flush();
    output.Position = 0;

    return File(output, "application/vnd.ms-excel", "test.xls");

}

when am trying to open excel i get this error

the file you are trying to open is in a different format than
specified by the file extension

After clicking on Yes the file open properly. but i don’t want this msg to appear.

Why this Range / Worksheet targeting trips people up

The question centers on reaching a specific cell, range, or workbook object. In Excel VBA, this is the #1 source of failures after activation events: every property (.Value, .Formula, .Address) behaves differently depending on whether the parent Workbook is explicit or implicit.

The Verified Solution — strong answer (top 25 %%) (+19)

19-line Excel VBA pattern (copy-ready)

I have used CloseXML to solve the problem.

public static void ExportToExcel(IEnumerable<dynamic> data, string sheetName)
{
    XLWorkbook wb = new XLWorkbook();
    var ws = wb.Worksheets.Add(sheetName);
    ws.Cell(2, 1).InsertTable(data);
    HttpContext.Current.Response.Clear();
    HttpContext.Current.Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
    HttpContext.Current.Response.AddHeader("content-disposition", String.Format(@"attachment;filename={0}.xlsx",sheetName.Replace(" ","_")));

    using (MemoryStream memoryStream = new MemoryStream())
    {
        wb.SaveAs(memoryStream);
        memoryStream.WriteTo(HttpContext.Current.Response.OutputStream);
        memoryStream.Close();
    }

    HttpContext.Current.Response.End();
}

Installed ClosedXML in my project using Nuget Package Manager.

When to Use It — classic (2013–2016)

Ranked #44th in its category — specialized fit

This pattern sits in the 94% tail relative to the top answer. Reach for it when your scenario closely matches the question title; otherwise browse the Excel VBA archive for a higher-consensus alternative.

What changed between 2013 and 2026

The answer is 13 years old. The Excel VBA object model has been stable across Office 2013, 2016, 2019, 2021, 365, and 2024/2026 LTSC, so the pattern still compiles. Changes that might affect you: 64-bit API declarations (use PtrSafe), blocked macros in downloaded files (Mark-of-the-Web), and the shift toward Office Scripts for web-first workflows.