如何使用 Entity Framework 连接三个表来获取所有数据

Question

我收到这个错误：

System.NullReferenceException: Object reference not set to an instance of an object

如何解决这个异常？

加入查询控制器：

var Cs = new List<MyModel>();

using (Join3Entities1 db = new Join3Entities1())
{
    DateTime start1 = DateTime.Now;
    ViewBag.Start = "Start Time :" + start1;
    Cs = (from e in db.Students
          join p in db.Marks on e.S_ID equals p.S_ID
          join t in db.Details on p.School_ID equals t.School_ID
          where p.Score > 50
          select new MyModel
                  {
                      S_Name = e.S_Name,
                      Score = (int)p.Score,
                      Status = p.Status,
                      Address_City = t.Address_City,
                      Email_ID = t.Email_ID,
                      Accomplishments = t.Accomplishments
                  }).ToList();
               DateTime end1 = DateTime.Now;
               ViewBag.end = "End Time:" + end1;
               TimeSpan time1 = end1 - start1;
               ViewBag.time = "TimeSpan:" + time1;
        }
        return View();

上面的代码是加入三个table我写在controller部分

型号： public class 我的模型 { public 字符串 S_Name { 得到；放; } public int Score { get;放; } public 字符串状态 { 得到;放; } public 字符串 Address_City { 得到；放; } public 字符串 Email_ID { 得到；放; } public 字符串成就 { get;放; } }

查看：

@model IEnumerable<Join3table.Models.MyModel>
@{
   ViewBag.Title = "Home Page";
 }

@foreach (var per in Model)
{
    <tr>
        <td>@per.S_Name</td>
        <td>@per.Score</td>
        <td>@per.Status</td>
        <td>@per.Address_City</td>
        <td>@per.Email_ID </td>
        <td>@per.Accomplishments</td>
    </tr>
}

</tbody>
</table>

我用主键和外键关系创建了三个 tables student,mark 和 details

Answer 1

而不是 select new model，尝试一个一个地返回每个 table 以查看 nullref 具体来自哪里，这样您就可以缩小范围。

另一个安全的选择是确保在调用每个列之前进行 nullrefcheck S_Name = e != null && !string.isNullOrEmpty(e.S_Name) ? e.S_Name : string.Empty;

如何使用 Entity Framework 连接三个表来获取所有数据

How to fetch all data using join three tables using Entity Framework

c#

asp.net-mvc

razor

entity-framework-6