how to get html dropdown selected value in C#.net ,asp.net

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ASP.NET

C#3.5

SQL-Server-2008R2

my code is

protected void Page_Load(object sender, EventArgs e)
   {
       if (!Page.IsPostBack)
       {
           DataSet ds = getdata();
           StringBuilder sb = new StringBuilder();
           sb.AppendLine("<table>");
           sb.AppendLine("<tr>");
           sb.Append("<td>");
           sb.Append("<select id='drp1'>");
           sb.Append("<option value='0'> select </option>");
           for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
           {
    sb.Append("<option value='" + ds.Tables[0].Rows[i]["id"].ToString() + "'> " + ds.Tables[0].Rows[i]["name"].ToString() + "</option>");              

           }
           sb.Append("</select>");
           sb.AppendLine("</table>");
           sb.AppendLine("</tr>");
           sb.Append("</td>");
           d1.InnerHtml = sb.ToString();
           var page = (Context.Handler as System.Web.UI.Page);
       }
   }
   public DataSet getdata()
   {
       using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["sqlcon"].ToString()))
       {
           SqlDataAdapter da = new SqlDataAdapter("select id,name from empdetails", con);
           DataSet ds = new DataSet();
           da.Fill(ds);
           return ds;
       }
   }
   protected void btnadd_click(object sender, EventArgs e)
   {
       try
       {
          int h = Convert.ToInt32(Request.Form["drp1"]);
       }
       catch (Exception ex)
       {
       }
   }

when i am select any item and click on button ,in button click i wrote the code but it not getting selected value ,its showing in 'h' value is 0(zero).pls i want selected value.how to get pls respond my question .........