No.1实现登陆功能，验证用户名和密码。从数据库里进行匹配，看是否有符合要求的数据。
在DAL层编写代码，返回值为布尔类型。方法参数为（student实体类对象），使用参数化SqlParameter类型防止SQL注入。
1 public bool IsLogin(Student stu)
2 {
3 bool flag = false;
4 SqlConnection con = new SqlConnection(SqlHelper.str);
5
6 string sql = "select count(1) from student where studentNo=@StudentName and LoginPwd=@LoginPwd";
7 //02 将sql语句交给SQL服务器执行
8 SqlParameter para1 = new SqlParameter("@StudentName", stu.StudentNo);
9 SqlParameter para2 = new SqlParameter("@LoginPwd", stu.LoginPwd);
10 SqlCommand cmd = new SqlCommand(sql,con);
11 cmd.Parameters.Add(para1);
12 cmd.Parameters.Add(para2);
13 try
14 {
15 con.Open();
16 int count = Convert.ToInt32(cmd.ExecuteScalar());
17 if (count > 0)
18 {
19 flag = true;
20 }
21 }
22 catch (Exception ex)
23 {
24
25 throw ex;
26 }
27 finally
28 {
29 con.Close();
30 }
31 return flag;
32 }
在BLL层创建DAL层的对象，调用DAL层的login方法，返回值和参数和DAL层login方法一致，return dal.login(stu)
1 public bool IsLogin(Student stu)
2 {
3 return dal.IsLogin(stu);
4 }
在UI层调用BLL层的方法，并在UI层创建实体类对象，给实体类对象的属性赋值后将对象放入调用的BLL层的方法中。判断登陆是否成功！
1 StudentBLL bll = new StudentBLL();
2 Student stu = new Student();
3 stu.StudentNo = Convert.ToInt32(txtName.Text);
4 stu.LoginPwd = txtPwd.Text;
5 bool flag = bll.IsLogin(stu);
6 if (flag)
7 {
8 MessageBox.Show("登陆成功");
9 frmlist list = new frmlist();
10 list.Show();
11 }
12 else
13 {
14 MessageBox.Show("登陆失败");
15 }
No.2展示学生信息
在DAL层编写方法返回值是泛型集合，无方法参数，使用工具类（SqlHelper类）将DataTable类型转换为泛型
1 public List<Student> Select()
2 {
3 SqlConnection con = new SqlConnection(SqlHelper.str);
4 string sql = "select * from student";
5 DataTable dt = SqlHelper.ExecuteDataTable(sql);
6 My_Tool tool = new My_Tool();
7 List<Student> list = tool.DataTableToList<Student>(dt);
8 return list;
9
10 }
在BLL层编写方法与DAL层同理
1 public List<Student> Select()
2 {
3 return dal.Select();
4 }
在UI层的初始化Select方法（）将数据绑定到datagridView中！
1 private void frmlist_Load(object sender, EventArgs e)
2 {
3 Initial();
4 }
5 public List<Student> list = new List<Student>();
6 public void Initial()
7 {
8 StudentBLL bll = new StudentBLL();
9 list= bll.Select();
10 this.dgvlist.DataSource = list;
11
12 }
No.3实现添加学生信息 
Insert语句插入数据的参数较多，所以我们可以声明一个SqlParaMeters类型的数组，节约代码，方法返回值依然是bool类型，方法参数是Student类型的对象。注意！在给Command对象添加参数的时候
要使用AddRange()方法！
1 public bool Insert(Student stu)
2 {
3 bool flag = false;
4 SqlConnection con = new SqlConnection(SqlHelper.str);
5 string sql = "insert into student values(@LoginPwd,@StudentName,@Gender,@GradeId,@Phone,@Address,@Birthday,@Email,@MyTT)";
6 SqlParameter[] para =
7 {
8 new SqlParameter("@LoginPwd", stu.LoginPwd),
9 new SqlParameter("@StudentName", stu.StudentName),
10 new SqlParameter("@Gender", stu.Gender),
11 new SqlParameter("@GradeId", stu.GradeId),
12 new SqlParameter("@Phone", stu.Phone),
13 new SqlParameter("@Address", stu.Address),
14 new SqlParameter("@Birthday", stu.Birthday),
15 new SqlParameter("@Email", stu.Email),
16 new SqlParameter("@MyTT", stu.MyTT)
17 };
18 SqlCommand cmd = new SqlCommand(sql,con);
19 cmd.Parameters.AddRange(para);
20
21 try
22 {
23 con.Open();
24 int count = Convert.ToInt32(cmd.ExecuteScalar());
25 if (count > 0)
26 {
27 flag = true;
28 }
29
30 }
31 catch (Exception ex)
32 {
33
34 throw ex;
35 }
36 finally
37 {
38 con.Close();
39 }
40 return flag;
41 }
在BLL层同样实现了方法调用！
1 public bool Insert(Student stu)
2 {
3 return dal.Insert(stu);
4 }
在UI层调用BLL层的方法，给Student类的属性赋值，判断是否添加学生信息成功！
1 StudentBLL bll = new StudentBLL();
2 Student stu = new Student();
3
4 stu.LoginPwd = txtPwd.Text;
5 stu.StudentName = txtName.Text;
6 if (rbtnFemale.Checked)
7 {
8 stu.Gender = "0";
9 }
10 else if (rbtnMan.Checked)
11 {
12 stu.Gender = "1";
13 }
14 if (cboGrade.SelectedItem.ToString() == "S1")
15 {
16 stu.GradeId = 0;
17 }
18 else if (cboGrade.SelectedItem.ToString() == "S2")
19 {
20 stu.GradeId = 1;
21 }
22 stu.Phone = txtPhone.Text;
23 stu.Address = txtAddress.Text;
24 stu.Birthday = dpBirthday.Value;
25 stu.Email = txtEmail.Text;
26 this.listnew.Add(stu);
27 bool flag= bll.Insert(stu);
28 frm.dgvlist.DataSource = new BindingList<Student>(this.listnew);
29 this.Close();
No.4实现comboBox的两级联动，当选中下拉框中的一项时，另一个下拉框会自动匹配，并且以筛选条件来查询！ 
在GradeDAL层编写读取所有年级下拉框的方法，返回值为泛型集合，无方法参数
1 public List<Grade> GetGradenew()
2 {
3 List<Grade> list = new List<Grade>();
4 using (SqlConnection con = new SqlConnection(SqlHelper.str))
5 {
6 string sql = "select * from Grade";
7 SqlCommand cmd = new SqlCommand(sql,con);
8 con.Open();
9 using (SqlDataReader dr = cmd.ExecuteReader())
10 {
11 while (dr.Read())
12 {
13 Grade ga = new Grade();
14 ga.GradeId = Convert.ToInt32(dr["GradeId"]);
15 ga.GradeName = dr["GradeName"].ToString();
16 list.Add(ga);
17
18 }
19 }
20
21 }
22 return list;
23 }
在BLL层调用该方法！
1 public List<Grade> GetGradenew()
2 {
3 return dal.GetGradenew();
4 }
在UI 层中调用此方法，comboBox的disPalyMember属性为“gradename”，valuemember属性设置为“gradeid”，将comboBox的DropDownHeight属性设置为106，避免下拉框冗余
1 this.comboBox2.DropDownHeight = 106;
2 this.comboBox1.DataSource = bll.GetGradenew();
3 this.comboBox1.ValueMember = "GradeId";
4 this.comboBox1.DisplayMember = "GradeName";
在DAL层编写两个方法，返回值都是泛型，一个方法有返回值，另一个没有，一个方法是为了查询所有科目，另一个是根据年级编号查询科目
public List<Subject> GetSubjectName()
{
List<Subject> list = new List<Subject>();
using (SqlConnection con = new SqlConnection(SqlHelper.str))
{
string sql = "select * from Subject";
SqlCommand cmd = new SqlCommand(sql, con);
con.Open();
using (SqlDataReader dr = cmd.ExecuteReader())
{
while (dr.Read())
{
Subject sub = new Subject();
sub.SubjectId = Convert.ToInt32(dr["SubjectId"]);
sub.SubjectName = dr["SubjectName"].ToString();
sub.GradeId = Convert.ToInt32(dr["GradeId"]);
sub.ClassHour = Convert.ToInt32(dr["ClassHour"]);
list.Add(sub);
}
}
}
return list;
}
public List<Subject> GetSubjectByGrade(int gradeid)
{
List<Subject> list = new List<Subject>();
using (SqlConnection con = new SqlConnection(SqlHelper.str))
{
string sql = "select * from Subject where gradeId='" + gradeid + "'";
SqlCommand cmd = new SqlCommand(sql, con);
con.Open();
using (SqlDataReader dr = cmd.ExecuteReader())
{
while (dr.Read())
{
Subject sub = new Subject();
sub.SubjectId = Convert.ToInt32(dr["SubjectId"]);
sub.SubjectName = dr["SubjectName"].ToString();
sub.GradeId = Convert.ToInt32(dr["GradeId"]);
sub.ClassHour = Convert.ToInt32(dr["ClassHour"]);
list.Add(sub);
}
}
}
return list;
}
在BLL层调用方法
1 SubjectDAL dal = new SubjectDAL();
2 public List<Subject> GetSubjectName()
3 {
4 return dal.GetSubjectName();
5 }
6 public List<Subject> GetSubjectByGrade(int gradeid)
7 {
8 return dal.GetSubjectByGrade(gradeid);
9 }
10 public List<Subject> GetSubjectById(int subjectid)
11 {
12 return dal.GetSubjectById(subjectid);
13 }
在UI层编写代码，注意如果其他事件触发的顺序在窗体load事件之前，使用bool类型变量来调整先后顺序，执行完load事件后flag的值为ture，如果为True，执行其他的事件，在科目下拉框再加入一个请选择，用泛型的Insert方法将其加到下标为0的位置上！
1 private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
2 {
3 if (flag)
4 {
5 int num = Convert.ToInt32(this.comboBox1.SelectedValue);
6
7 List < Subject > list= sub.GetSubjectByGrade(num);
8 Subject ject = new Subject();
9 ject.SubjectId = -1;
10 ject.SubjectName = "请选择";
11 list.Insert(0, ject);
12 this.comboBox2.ValueMember = "SubjectId";
13 this.comboBox2.DisplayMember = "SubjectName";
14 this.comboBox2.DataSource = list;
15 }
16 }
在DAL层编写方法，作为条件查询学生成绩的方法，根据查询条件来设置方法参数和SQL语句，实现多种条件下的模糊查询
1 public DataTable GetList(string name, int id)
2 {
3 using (SqlConnection con = new SqlConnection(SqlHelper.str))
4 {
5 string sql = "select StudentName,subjectName,studentresult,examdate from student,result,Subject,grade where subject.gradeid=grade.GradeId and result.studentNo=student.studentNo and result.subjectid=subject.subjectid";
6 if (id != -1)
7 {
8 sql += " and Subject.Subjectid='" + id + "'";
9 }
10 if (!string.IsNullOrEmpty(name))
11 {
12 sql += " and StudentName='" + name + "'";
13 }
14 SqlDataAdapter da = new SqlDataAdapter(sql, con);
15 DataSet ds = new DataSet();
16 da.Fill(ds, "ss");
17 return ds.Tables["ss"];
18 }
19 }
在BLL层调用该方法！
1 public DataTable GetList(string name, int id)
2 {
3 return dal.GetList(name, id);
4 }
在UI层调用该方法，给方法参数赋值，实体类的属性赋值
1 private void button1_Click(object sender, EventArgs e)
2 {
3 string name = textBox1.Text;
4 int subjectid = Convert.ToInt32(comboBox2.SelectedValue);
5 DataTable dt = re.GetList(name, subjectid);
6 this.dataGridView1.DataSource = dt;
7 }