Down load file from Solution Explorar

protected void Button2_Click(object sender, EventArgs e)
    {
        string s = DropDownList1.SelectedItem.Text;
        Response.Clear();
        Response.AddHeader("content-disposition", string.Format("attachment;filename={0}", s));
        Response.Charset = "";
        Response.ContentType = "application/vnd.xls";

        StringWriter stringWrite = new StringWriter();
        HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
    
        Response.Write(stringWrite.ToString());
        Response.End();
    }

Retrive the files For Dropdownlist

 protected void Page_Load(object sender, EventArgs e)
    {
        retrivefiles();
    }
    public void retrivefiles()
    {
      
        DirectoryInfo dinfo = new DirectoryInfo(Server.MapPath(("." + @"\images\")));
        FileInfo[] fileslist = dinfo.GetFiles();
        foreach (FileInfo fi in fileslist)
        {
            DropDownList1.Items.Add(fi.Name);
        }
    }