Excel import and rendering in html control

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C#3.0

C#

ASP.NET

After clicking on some button the page, i run some function to fetch the records in dg and pass dg to below function and filename.

private void ExportToExcel(string strFileName, DataGrid dg)
    {

HttpContext.Current.Response.ClearContent();
        HttpContext.Current.Response.Clear();
        HttpContext.Current.Response.Buffer = true;

        HttpContext.Current.Response.AddHeader("content-disposition", "attachment; filename=" + strFileName + ".xls");
        HttpContext.Current.Response.ContentType = "application/vnd.ms-excel";

        HttpContext.Current.Response.Charset = "";
        HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.NoCache);
        
        System.IO.StringWriter sw = new System.IO.StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
//dg is DataGrid records
        dg.RenderControl(htw);
        HttpContext.Current.Response.Write(sw.ToString());
        HttpContext.Current.Response.End();}

when it promts to open the excel file then it shows below message

"The file you trying to open, 'filename.xls', is in a different format than specified by the file extension.Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?"<br />
Yes | No | Help