Don’t Export Persian fields finely from ASP.NET to Excel (VS2010 – C#)

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ASP.NET

Hi I export my GrideView from asp.net to excel but the fields which contain Persian character don’t export finely (VS2010 – C#). HOW CAN I DO IT?
My code is :

HttpContext.Current.Response.Clear();
    HttpContext.Current.Response.AddHeader("content-disposition",
        string.Format("attachment;filename={0}.xls", "Exported"));
    HttpContext.Current.Response.Charset = "";
    HttpContext.Current.Response.ContentType = "application/ms-excel";

    using (StringWriter sw = new StringWriter())
    {
        using (HtmlTextWriter htw = new HtmlTextWriter(sw))
        {
            //  Create a table to contain the grid
            Table table = new Table();

            foreach (GridViewRow row in GridView1.Rows)
            {
                table.Rows.Add(row);
            }

            //  render the table into the htmlwriter
            table.RenderControl(htw);

            //  render the htmlwriter into the response
            HttpContext.Current.Response.Write(sw.ToString());
            HttpContext.Current.Response.End();
        }
    }

Thanks very much