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;
}