ok solved :) put connection string in code behind fix the problem if is in "If(!IsPostBack):
if (!IsPostBack)
{
SqlCommand cmd = new SqlCommand("Select [CountryId], [CountryName] from [Country]", conn);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
conn.Open();
this.ddlcountry.DataSource = ds.Tables[0];
this.ddlcountry.DataBind();
conn.Close();
}