c#ComboBox的联动

StudentDAL

public class StudentDAL
    {
        public List SelectGradeName()
        {
            string str = SqlHelper.Constr;        
            string sql = "select * from grade";
            List glist = new List();
            using (SqlConnection con = new SqlConnection(str))
            {
                SqlCommand cmd = new SqlCommand(sql,con);
                con.Open();
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    Grade item = new Grade();
                    item.GradeId = Convert.ToInt32(dr["gradeid"]);
                    item.GradeName = dr["gradename"].ToString();
                    glist.Add(item);
                }
                dr.Close();
                return glist;
            }
         }
        
        public List SelectSubjectName(string name)
        {
            string str = SqlHelper.Constr;
            string sql = "select * from subject,grade where grade.gradeid=subject.gradeid and gradename='"+name+"'";
            List slist = new List();
            using (SqlConnection con = new SqlConnection(str))
            {
                SqlCommand cmd = new SqlCommand(sql, con);
                con.Open();
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    Subject item = new Subject();
                    item.SubjectId = Convert.ToInt32(dr["subjectid"]);
                    item.SubjectName = dr["subjectname"].ToString();
                    item.ClassHour = Convert.ToInt32(dr["classhour"]);
                    item.GradeId = Convert.ToInt32(dr["gradeid"]);
                    slist.Add(item);
                }
                dr.Close();
                return slist;
            }
        }
        public List SelectSubjectName()
        {
            string str = SqlHelper.Constr;
            string sql = "select * from subject";
            List slist = new List();
            using (SqlConnection con = new SqlConnection(str))
            {
                SqlCommand cmd = new SqlCommand(sql, con);
                con.Open();
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    Subject item = new Subject();
                    item.SubjectId = Convert.ToInt32(dr["subjectid"]);
                    item.SubjectName = dr["subjectname"].ToString();
                    item.ClassHour = Convert.ToInt32(dr["classhour"]);
                    item.GradeId = Convert.ToInt32(dr["gradeid"]);
                    slist.Add(item);
                }
                dr.Close();
                return slist;
            }
        }

        public DataTable FillListView(string name,string grade,string subject)
        {
            string str = SqlHelper.Constr;
            using(SqlConnection con = new SqlConnection(str)){
            string sql = "select studentname as '学生姓名',subjectname as '科目',studentresult as '成绩',examdate as '考试时间' from result inner join student on student.studentno=result.studentno inner join subject on subject.subjectid=result.subjectid inner join grade on grade.gradeid=student.gradeid where studentname like '%" + name + "%' and gradename='"+grade+"' and subjectname='"+subject+"'";
            con.Open();
            SqlDataAdapter da = new SqlDataAdapter();
            SqlCommand cmd = new SqlCommand(sql, con);
            da.SelectCommand = cmd;
            DataSet ds = new DataSet();
            da.Fill(ds, "Student");
            con.Close();
            return ds.Tables["Student"];
            }
        }
    }

winform窗体

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using MySchoolDAL;
using MySchoolModels;

namespace MySchoolUI
{
    public partial class StudentResult : Form
    {
        public StudentResult()
        {
            InitializeComponent();
        }

        private void btnSearch_Click(object sender, EventArgs e)
        {
            StudentDAL sd = new StudentDAL();
            string s1 = this.comboBox1.Text;
            string s2 = this.comboBox2.Text;
            string name = this.txtStudentName.Text;
            DataTable dt = sd.FillListView(name,s1,s2);
            this.dataGridView1.DataSource = dt;
        }
            StudentDAL sd = new StudentDAL();
        private void StudentResult_Load(object sender, EventArgs e)
        {
            List lg = sd.SelectGradeName();
            foreach (Grade item in lg)
            {
                this.comboBox1.Items.Add(item.GradeName);
            }
            List ls = sd.SelectSubjectName();
            foreach (Subject item in ls)
            {
                this.comboBox2.Items.Add(item.SubjectName);
            }
        }

        private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
        {

        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            string s = this.comboBox1.Text;
            this.comboBox2.Items.Clear();
            List ls = sd.SelectSubjectName(s);
            foreach (Subject item in ls)
            {
                this.comboBox2.Items.Add(item.SubjectName);
            }
        }
    }
}