如何从Dropdownlist中选择值时自动填充文本框

以下代码用于从数据库加载下拉列表。但是我希望从文本框中的下拉列表中显示所有数据的选定数据。

following code is for load dropdown list from database. But i want to show a selected data from dropdownlist in textbox with all deails.

protected void load_customer()
{
   using (MySqlConnection con = new MySqlConnection(constr))
   {
      using (MySqlCommand cmd = new MySqlCommand("SELECT concat(firstname, ' ', lastname) as FullName, CustNo FROM tblcustomer order by CustNo"))
      {
         cmd.CommandType = CommandType.Text;
         cmd.Connection = con;
         con.Open();
         ddCustomer.DataSource = cmd.ExecuteReader();
         ddCustomer.DataTextField = "FullName";
         ddCustomer.DataValueField = "CustNo";
         ddCustomer.DataBind();
         con.Close();
      }
   }
   ddCustomer.Items.Insert(0, new ListItem("--Select Customer--", "0"));
}