You can use namespace
using System.Data.SqlClient;
using System.Data;
and write code and call the getID method()
public void getID()
{
SqlDataAdapter adp = new SqlDataAdapter("select ID From Student", con);
DataSet ds = new DataSet();
adp.Fill(ds);
DropDD.DataValueField = "ID";
DropDD.DataSource = ds;
DropDD.DataBind();
}