c# - ASP.net Gridview Excel .xlsx へのエクスポートが機能しない

Question

.xls での Excel へのエクスポートは機能していますが、コンテンツ タイプの変更後に .xlsx へのエクスポートが機能しません。私のコードは以下のとおりです。

private void ExportToExcel()
{
    try
    {
        Response.Clear();
        Response.Buffer = true;

        //Response.AddHeader("content-disposition", "attachment;filename=LoanDataDeletion.xls");
        //Response.Charset = "";
       // Response.ContentType = "application/vnd.ms-excel";

        Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
        Response.Charset = "";
        Response.AddHeader("Content-Disposition", string.Format("attachment; filename={0}", "LoanDataDeletion.xlsx"));

        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);

        grdView.AllowPaging = false;
        grdView.DataBind();

        //Change the Header Row back to white color
        grdView.HeaderRow.Style.Add("background-color", "#FFFFFF");

        //Apply style to Individual Cells
        for (int i = 0; i < grdView.Columns.Count; i++)
        {
            grdView.HeaderRow.Cells[i].Style.Add("background-color", "green");
        }
        for (int i = 0; i < grdView.Rows.Count; i++)
        {
            GridViewRow row = grdView.Rows[i];

            //Change Color back to white
            row.BackColor = System.Drawing.Color.White;

            //Apply text style to each Row
            row.Attributes.Add("class", "textmode");

            //Apply style to Individual Cells of Alternating Row
            if (i % 2 != 0)
            {
                row.Cells[0].Style.Add("background-color", "#C2D69B");
                row.Cells[1].Style.Add("background-color", "#C2D69B");
                row.Cells[2].Style.Add("background-color", "#C2D69B");
                row.Cells[3].Style.Add("background-color", "#C2D69B");
            }
        }
        grdView.RenderControl(hw);

        //style to format numbers to string
        string style = @"<style> .textmode { mso-number-format:\@; } </style>";
        Response.Write(style);
        Response.Output.Write(sw.ToString());
        Response.Flush();
        Response.End();
    }
    catch (Exception)
    {
        throw;
    }
}