ComboBox的联动（三层架构）

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(, new Grade() { GradeId=-,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(, new Grade() { GradeId=-,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");
            }