C# ComboBox的联动操作（三层架构）

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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