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");

            }