how to bind drop down lists in c#?

Question

private void comboBoxComNS_SelectedIndexChanged(object sender, EventArgs e)//country

{

ModelBind();

string str = Convert.ToString(comboBoxComNS.SelectedValue);

SqlCommand cmd = new SqlCommand("Select MODELNUM from TBL_MODEL where COMPID=@compid", conn);

cmd.Parameters.AddWithValue("@compid", str);

SqlDataAdapter da = new SqlDataAdapter(cmd);

DataSet ds = new DataSet();

da.Fill(ds, "Modelnew");

comboBoxMNS.DataSource = ds.Tables["Modelnew"];

}