Question

应用程序中的服务器错误。

对象引用未设置为对象的实例。

描述：执行期间发生了未处理的异常   当前的网络请求。请查看堆栈跟踪   有关错误及其来源的更多信息   码。

异常详细信息：System.NullReferenceException：不是对象引用   设置为对象的实例。

Source Error: 


Line 29:         <th></th>
Line 30:     </tr>
Line 31:     @foreach (var sections in Model.Sections)
Line 32:     {
Line 33:         <tr>

我的模特

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace TechFactorsLMSV2.Models
{

public class School
{
    public int ID { get; set; }
    public string SchoolName { get; set; }
    public ICollection<Section> Sections { get; set; }
}
public class Section
{
    public int ID { get; set; }
    public string SectionName { get; set; }
    public ICollection<Student> Students { get; set; }
}

public class Student
{
    public int ID { get; set; }
    public string LastName { get; set; }
    public string FirstName { get; set; }
    public string MiddleName { get; set; }
    public string Address { get; set; }
    public DateTime DateEnrolled { get; set; }

}

public class LMSDBContext : DbContext
{
    public DbSet<School> Schools { get; set; }
    public DbSet<Section> Sections { get; set; }
    public DbSet<Student> Students { get; set; }
}
}

我的控制器

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.Entity;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using TechFactorsLMSV2.Models;

namespace TechFactorsLMSV2.Controllers
{
public class SchoolsController : Controller
{
    private LMSDBContext db = new LMSDBContext();

    //
    // GET: /Schools/

    public ActionResult Index()
    {
        return View(db.Schools.ToList());
    }
public ActionResult Detail(int id)
    {
        var model = db.Schools.Single(d => d.ID == id);
        return View(model);
    }
 } 
}

我的观点

 TechFactorsLMSV2.Models.School

@{
ViewBag.Title = "Detail";
}

<h2>@Model.SchoolName</h2>

@Html.ActionLink("Add Section", "Create", "Section", new { SchoolId = @Model.ID }, null      

}

<table>
  <tr>
    <th>Sections</th>
    <th></th>
  </tr>
  @foreach (var sections in Model.Sections)
{
    <tr>
        <td>@sections.SectionName</td>
        <td>


        </td>

    </tr>
 }


</table>

Answer 1

我建议阅读lazy / eager loading。您的部分根本不会加载到模型中......

Answer 2

如果你向学校班级添加一个构造函数，那么它就会消除这个例外;例如，

public class School
{
    public int ID { get; set; }
    public string SchoolName { get; set; }
    public ICollection<Section> Sections { get; set; }

    public School()
    {
        Sections = new List<Section>();
    }
}

但是，正如@Peter所说，你需要考虑如何填充，以及通过所有不是为了SO。

我有和Object引用中的错误没有设置为对象的实例

2 个答案: