項目需求：根據年級下拉框的變化使得科目下拉框綁定次年級下對應有的值
我們用三層架構的模式來實現
1.我們想和數據庫交互，我們首先得來先解決DAL數據庫交互層
01.獲得年級下拉框的數據
在GradeDAL類中
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{ //數據訪問層 public class GradeDAL { public static string Constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString; #region 獲得年級表 public DataTable SelectGrade(string gradetype) { //和數據庫交互 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(); //捕獲異常 try { da.Fill(ds, "stuInfo"); } catch (Exception ex) { throw new Exception(ex.Message); } //返回一張表的數據 return ds.Tables["stuInfo"]; } #endregion #region 獲取年級數據，為在下拉框中顯示 //定義一個集合,儲存年級信息 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) { //每一個row代表表中的一行,所以一行對應一個年級對象 Grade grade = new Grade(); grade.GradeId = Convert.ToInt32(row["gradeid"]); grade.GradeName = row["gradename"].ToString(); list.Add(grade); } //方法二：(使用MyTool類) //MyTool tool=new MyTool(); //list = tool.DataTableToList<Grade>(dt); return list; } #endregion #region 方法三：要求使用using語句 public List<Grade> LoadCombox3() { //using的作用可以釋放資源，利于資源的回收（可以省略關閉連接） 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.在業務邏輯層
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 獲取年級數據，為在下拉框中顯示 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.在窗體UI層
在Load事件中加載年級下拉框
private void FrmSelectResult_Load(object sender, EventArgs e) { #region 加載年級下拉框 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 加載科目下拉框 //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
獲得年級下拉框隱藏值得方法（2）
int num = Convert.ToInt32(cboGrade.SelectedValue);
加載年級下拉框時：會出現的錯誤的寫法
把cboGrade.DataSource = list;寫在
cboGrade.ValueMember = "GradeId";
cboGrade.DisplayMember = "GradeName";上面
即：
#region 加載年級下拉框 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 這是就會出現下面錯誤：
在年級的SelectedIndexChanged事件中
try { //根據年級取得科目信息并綁定 #region 獲得年級下拉框隱藏值得方法（1） Grade sub = (Grade)cboGrade.SelectedItem; int num =sub.GradeId; #endregion #region 獲得年級下拉框隱藏值得方法（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("出錯"); } 以上就是本文的全部內容，希望對大家學習C#程序設計有所幫助。
