objConnection.Open();
            string sql = "select StudentInfo.StudentId as 自动编号,studentinfo.studentnumber as 学号,studentinfo.name as 姓名,studentinfo.sex as 性别,studentinfo.race as 族别,studentinfo.address as 家庭住址,studentinfo.telephone as 联系方式,studentinfo.role as 政治面貌,studentinfo.postalcode as 邮政编码,classinfo.classname as 班级名称,studentinfo.remark as 备注  from classinfo,studentinfo where classinfo.classid=studentinfo.classid and classinfo.classname='" + comboBox2.SelectedItem.ToString().Trim() + "'";
            OleDbDataAdapter adp = new OleDbDataAdapter(sql, objConnection);
            DataSet das = new DataSet();
            das.Clear();
            adp.Fill(das, "class");
            
           dataGridView1.DataSource = das.Tables[0].DefaultView;
            objConnection.Close();

把错误代码贴出来看看

DataSet das = new DataSet();
adp.Fill(das, "class");
          dataGridView1.DataSource = das.Tables["class"]DefaultView;
            objConnection.Close(); 

去掉das.clear();

可能是你classinfo.classid=studentinfo.classid and classinfo.classname='" + comboBox2.SelectedItem.ToString().Trim() + "'等号两边定义的数据类型不同

非常感谢！  是数据类型不匹配， 谢谢大哥指点。  小弟我看了 一上午都没看出来那儿错了！  不过 查询结果 竟然为空 ，  很郁闷。