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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