how to populate data in dropdowlist from database fom different tables?

BusinessLayer.State objstate = new BusinessLayer.State(System.Configuration.ConfigurationManager.ConnectionStrings["test"].ToString());
    BusinessLayer.cities objcity = new BusinessLayer.cities(System.Configuration.ConfigurationManager.ConnectionStrings["test"].ToString());
    BusinessLayer.university objuni = new BusinessLayer.university(System.Configuration.ConfigurationManager.ConnectionStrings["test"].ToString());
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            Fillstate();
        }

    }
    public void Fillstate()
    {
     
        DataTable dt = new DataTable();
        dt = objstate.getdatastate();
        ddlstate.DataSource = dt;
        ddlstate.DataValueField = "State_Id";
        ddlstate.DataTextField = "State_Name";
        ddlstate.DataBind();
        ddlstate.Items.Add(new ListItem("- Please Select -", "-1"));
        ddlstate.SelectedValue = "-1";
        int state_id = Convert.ToInt32(ddlstate.SelectedIndex)+1;
        TextBox1.Text = state_id.ToString();

    }

    public void Fillcity(int state_id)
    {
        DataTable dt = new DataTable();
        dt = objcity.getdatacity(state_id);
        ddlcity.DataSource = dt;
        ddlcity.DataValueField ="City_Id";
        ddlcity.DataTextField ="City_Name";
        ddlcity.DataBind();
        ddlcity.Items.Add(new ListItem("- Please Select -", "-1"));
        ddlcity.SelectedValue ="-1";
      }
    public void Filluniversity(int city_id)
    {
        DataTable dt = new DataTable();
        dt = objuni.getdatauniversity();
        ddluni.DataSource = dt;
        ddluni.DataValueField = "Uni_Id";
        ddluni.DataTextField = "Uni_Name";
        ddluni.DataBind();
        ddluni.Items.Add(new ListItem("- Please Select -", "-1"));
        ddluni.SelectedValue = "-1";

    }

     protected void ddlstate_SelectedIndexChanged(object sender, EventArgs e)
    {
        int state_id = Convert.ToInt32(ddlstate.SelectedIndex)+1;
        Fillcity(state_id);
    }
    protected void ddlcity_SelectedIndexChanged(object sender, EventArgs e)
    {
        int city_id = Convert.ToInt32(ddlcity.SelectedValue.ToString());
        Filluniversity(city_id);
    }

public class cities
   {

        DataLibrary dl;

        public cities(string dbconnectionstring)
       {

           dl=new DataLibrary(dbconnectionstring);
       }
        public DataTable getdatacity(int state_id)
        {
            DataTable dt = new DataTable();
            dt = dl.GetDataTable("select City_Id,City_Name from City  where State_Id=state_id");
            return dt;
        }          }

public class DataLibrary
    {
       public SqlConnection Sqlconn;
       public SqlCommand sqlcmd;
       public SqlDataAdapter SqlAdp;
       public DataSet SqlDst;
       public DataTable SqlDtTbl;

        private string StrSqlconnection;

        public DataLibrary(String ConnectionString)    //Constructor declaration
        {
            StrSqlconnection = ConnectionString;
        }

        public int DMLQuery(string SqlCommandText)
        {
            
            Sqlconn = new SqlConnection(StrSqlconnection);
            Sqlconn.Open();
            sqlcmd = new SqlCommand();
            sqlcmd.CommandText = SqlCommandText;
            sqlcmd.Connection = Sqlconn;
            int i = sqlcmd.ExecuteNonQuery();
            return i;
        }

        public DataSet GetDataSet(string SqlCommandText)
        {
             SqlAdp = new SqlDataAdapter(SqlCommandText, StrSqlconnection);
             SqlDst = new DataSet();
            SqlAdp.Fill(SqlDst);
            return SqlDst;
        }

        public DataTable GetDataTable(string SqlCommandText)
        {
           SqlAdp = new SqlDataAdapter(SqlCommandText, StrSqlconnection);
            SqlDtTbl = new DataTable();
            SqlAdp.Fill(SqlDtTbl);
            return SqlDtTbl;
        }
    }