项目需求：根据年级下拉框的变化使得科目下拉框绑定次年级下对应有的值

我们用三层架构的模式来实现

1.我们想和数据库交互，我们首先得来先解决DAL数据库交互层

01.获得年级下拉框的数据

在GradeDAL类中

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;using System.Data.SqlClient;using System.Data;using MySchool.Model;using System.Configuration;namespace MySchool.DAL{ //数据访问层 public class GradeDAL { public static string Constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString; #region 获得年级表 public DataTable SelectGrade(string gradetype) { //和数据库交互 string str = "Data Source=.;initial catalog=MySchool;uid=sa"; SqlConnection con = new SqlConnection(str); string sql = ""; if (gradetype=="") { sql = "select * from Grade"; } else { sql = "select * from Student where GradeId in (select GradeId from Grade where GradeName='" + gradetype + "')"; } SqlDataAdapter da = new SqlDataAdapter(sql, con); DataSet ds = new DataSet(); //捕获异常 try { da.Fill(ds, "stuInfo"); } catch (Exception ex) { throw new Exception(ex.Message); } //返回一张表的数据 return ds.Tables["stuInfo"]; } #endregion #region 获取年级数据，为在下拉框中显示 //定义一个集合,储存年级信息 List<Grade> list = new List<Grade>(); #region 方法一: 以返回表的方式 public DataTable LoadCombox() { string sql = "select * from Grade"; DataTable dt = SQLHelper.ExecuteDataTable(sql); return dt; } #endregion #region 方法二:以返回集合的方式 public List<Grade> Loadcombox2() { string sql = "select * from Grade"; DataTable dt = SQLHelper.ExecuteDataTable(sql); //方法一： foreach (DataRow row in dt.Rows) { //每一个row代表表中的一行,所以一行对应一个年级对象 Grade grade = new Grade(); grade.GradeId = Convert.ToInt32(row["gradeid"]); grade.GradeName = row["gradename"].ToString(); list.Add(grade); } //方法二：(使用MyTool类) //MyTool tool=new MyTool(); //list = tool.DataTableToList<Grade>(dt); return list; } #endregion #region 方法三：要求使用using语句 public List<Grade> LoadCombox3() { //using的作用可以释放资源，利于资源的回收（可以省略关闭连接） using (SqlConnection con=new SqlConnection(Constr)) { try { string sql = "select * from Grade"; SqlCommand cmd = new SqlCommand(sql,con); con.Open(); SqlDataReader dr = cmd.ExecuteReader(); while (dr.Read()) { Grade gr = new Grade(); gr.GradeId = Convert.ToInt32(dr["GradeId"]); gr.GradeName=dr["GradeName"].ToString(); list.Add(gr); } } catch (Exception ex) { throw new Exception(ex.Message); } } return list; } #endregion #endregion }}

02.在业务逻辑层

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;using MySchool.DAL;using System.Data;using MySchool.Model;namespace MySchool.BLL{ public class GradeBLL { GradeDAL gradedal = new GradeDAL(); #region 获取年级数据，为在下拉框中显示 public DataTable SelectGrade(string gradetype) { return gradedal.SelectGrade(gradetype); } public DataTable LoadCombox() { return gradedal.LoadCombox(); } public List<Grade> Loadcombox2() { return gradedal.Loadcombox2(); } #endregion public List<Grade> LoadCombox3() { return gradedal.LoadCombox3(); } }}

03.在窗体UI层

在Load事件中加载年级下拉框

private void FrmSelectResult_Load(object sender, EventArgs e) { #region 加载年级下拉框 try { List<Grade> list = gradedal.LoadCombox3(); list.Insert(0, new Grade() { GradeId=-1,GradeName="--全部--" }); cboGrade.ValueMember = "GradeId"; cboGrade.DisplayMember = "GradeName"; cboGrade.DataSource = list; } catch (Exception ex) { MessageBox.Show(ex.Message); } #endregion #region 加载科目下拉框 //try //{ // list2 = subjectdal.LoadComboxSub(); // list2.Insert(0, new Subject() { SubjectId = -1, SubjectName = "--全部--" }); // cboSubject.ValueMember = "SubjectId"; // cboSubject.DisplayMember = "SubjectName"; // cboSubject.DataSource = list2; //} //catch (Exception ex) //{ // MessageBox.Show(ex.Message); //} #endregion }

其中在使用

获得年级下拉框隐藏值得方法（2）
int num = Convert.ToInt32(cboGrade.SelectedValue);

加载年级下拉框时：会出现的错误的写法

把cboGrade.DataSource = list;写在
cboGrade.ValueMember = "GradeId";
cboGrade.DisplayMember = "GradeName";上面
即：

#region 加载年级下拉框 try { List<Grade> list = gradedal.LoadCombox3(); list.Insert(0, new Grade() { GradeId=-1,GradeName="--全部--" }); cboGrade.DataSource = list; cboGrade.ValueMember = "GradeId"; cboGrade.DisplayMember = "GradeName"; } catch (Exception ex) { MessageBox.Show(ex.Message); } #endregion

这是就会出现下面错误：

在年级的SelectedIndexChanged事件中

try { //根据年级取得科目信息并绑定 #region 获得年级下拉框隐藏值得方法（1） Grade sub = (Grade)cboGrade.SelectedItem; int num =sub.GradeId; #endregion #region 获得年级下拉框隐藏值得方法（2） // int num = Convert.ToInt32(cboGrade.SelectedValue.ToString()); #endregion List<Subject> list = subjectdal.LoadComboxSub2(num); cboSubject.ValueMember = "SubjectId"; cboSubject.DisplayMember = "SubjectName"; cboSubject.DataSource = list; } catch (Exception) { MessageBox.Show("出错"); }

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