方法一：
   string sql="SELECT * FROM NewsClass ORDER BY ORDERID DESC";

   OleDbConnection conn=new OleDbConnection();

   conn.ConnectionString=ConnectionString;
   conn.Open();

   OleDbCommand cmd = new OleDbCommand(Sql,Conn);
   OleDbDataReader dr= cmd.ExecuteReader();
     //将数据添加到 DropDownList
   while(dr.Read())
   {
    this.ddlClass.Items.Add(new ListItem(dr["ClassName"].ToString(),dr["ClassId"].ToString()));
   }
   dr.Close();
   cmd.Dispose();
   conn.Close;
   conn.Dispose();

方法二：

  string sql="SELECT * FROM NewsClass ORDER BY ORDERID DESC";

   OleDbConnection conn=new OleDbConnection();
   conn.ConnectionString=ConnectionString;
   conn.Open();
 
   DataSet ds=new DataSet();
   OleDbDataAdapter da=new OleDbDataAdapter(sql,conn); 
   da.Fill(ds,"ClassTable");
   //将数据添加到 DropDownList
   this.ddlClass.DataSource=ds;
   this.ddlClass.DataTextField = "ClassName";
   this.ddlClass.DataValueField = "ClassID";
   this.ddlClass.DataBind();
   ds.Dispose();

   conn.Close;
   conn.Dispose();