擴展實體類的使用

需求：

上圖中，如果我們想用實體類來實現的話，明顯實現不了

原因很簡單，要顯示的數據包含的三張表（Student，Subject，Result）中的數據

我們無法單純的用某個實體類來解決，這時我們就可以擴展實體類來解決這個問題

我們可添加ResultBusiness類繼承Result實體類

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

namespace MySchool.Model
{
    //拓展類  繼承Result實體類
   public class ResultBusiness:Result
    {
       //多定義兩個屬性（達到目的）
       //學生姓名
       public string StudentName { get; set; }
       //科目名
       public string SubjectName { get; set; }
    }
}

這樣我們就可以使用ResultBusiness類來實現目的

方法：

public List<ResultBusiness> ReviewStudentResult2(int subjectId, string name)
      {
          List<ResultBusiness> list = new List<ResultBusiness>();
          // List<Result> list = new List<Result>();
          //創建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;
      }

調用：

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