扩展实体类的使用

需求：

上图中，如果我们想用实体类来实现的话，明显实现不了

原因很简单，要显示的数据包含的三张表（Student，Subject，Result）中的数据

我们无法单纯的用某个实体类来解决，这时我们就可以扩展实体类来解决这个问题

我们可添加ResultBusiness类继承Result实体类

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace MySchool.Model
{
    //拓展类  继承Result实体类
   public class ResultBusiness:Result
    {
       //多定义两个属性（达到目的）
       //学生姓名
       public string StudentName { get; set; }
       //科目名
       public string SubjectName { get; set; }
    }
}

这样我们就可以使用ResultBusiness类来实现目的

方法：

public List<ResultBusiness> ReviewStudentResult2(int subjectId, string name)
      {
          List<ResultBusiness> list = new List<ResultBusiness>();
          // List<Result> list = new List<Result>();
          //创建SQl
          StringBuilder sb = new StringBuilder();
          sb.Append("select * from Result,Student,Subject where Result.StudentNo=Student.studentNo and Subject.SubjectId=Result.SubjectId ");
          if (subjectId != -1)
          {
              sb.Append("and Result.SubjectId=@SubjectId ");
          }
          if (!string.IsNullOrEmpty(name))
          {

              sb.Append("and Student.StudentName like +  '%'+@Studentname+'%'");
          }
          SqlParameter[] para =
          {
              new SqlParameter("@SubjectId",subjectId),
              new SqlParameter("@Studentname",name)
          };
          SqlConnection con = new SqlConnection(Constr);
          SqlCommand cmd = new SqlCommand(sb.ToString(), con);
          cmd.Parameters.AddRange(para);
          con.Open();
          SqlDataReader dr = cmd.ExecuteReader();

          while (dr.Read())
          {

              ResultBusiness result = new ResultBusiness();
              result.StudentName = dr["StudentName"].ToString();
              result.SubjectName = dr["SubjectName"].ToString();
              result.StudentNo = Convert.ToInt32(dr["StudentNo"].ToString());
              result.StudentResult = Convert.ToInt32(dr["StudentResult"].ToString());
              result.SubjectId = Convert.ToInt32(dr["SubjectId"].ToString());
              result.ExamDate = Convert.ToDateTime(dr["ExamDate"].ToString());
              list.Add(result);
          }


          con.Close();
          dr.Close();
          return list;
      }

调用：

 public void Three()
        {
            string Name = txtName.Text;
            int grade = Convert.ToInt32(cboGrade.SelectedValue);
            List<ResultBusiness> list = resultbll.ReviewStudentResult2(grade, Name);
            dgvResult.DataSource = list;
        }