I am making a page and on this page, there is a table. I want to filter Name from the table. And I wrote code like this:

Index.cshtml:

@using StudentApp.Models.Entity
@model List<StudentTable>
@{
    ViewData["Title"] = "Manage Student";
    Layout = "~/Views/Shared/_Layout.cshtml";
}
<br />
<h1>Manage Student</h1>
<br />
@using (Html.BeginForm("Index", "Students", FormMethod.Get))
{
    <p>
        <b>Student Name:</b> @Html.TextBox("p");
        <input type="submit" value="Ara">
    </p>
}
<table id="tbl1" >
    <tr>
        <th>
            Student ID
        </th>
        <th>
            Student Name
        </th>
        <th>
            Student Class
        </th>
        <th>
            Edit
        </th>
        <th>
            Delete
        </th>
    </tr>
    <tbody>
        @foreach (var item in Model)
        {
            <tr>
                <td>
                    @item.Id
                </td>
                <td>
                    @item.Name
                </td>
                <td>
                    @item.Class.ClassName // Error
                </td>
                <td>
                    <a href="/StudentTable/EditStudent/@item.Id" >Edit</a>
                </td>
                <td>
                    <a href="/StudentTable/Delete/@item.Id" >Delete</a>
                </td>
            </tr>
        }
    </tbody>
</table>

<a href="/StudentTable/AddStudent"  >Add Student</a>


<script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<link href="~/lib/bootstrap/dist/css/bootstrap.css" rel="stylesheet" />
<link href="https://cdn.datatables.net/1.10.15/css/dataTables.bootstrap.min.css" rel="stylesheet" />
<link href="https://cdn.datatables.net/responsive/2.1.1/css/responsive.bootstrap.min.css" rel="stylesheet" />
<script src="https://cdn.datatables.net/1.10.15/js/jquery.dataTables.min.js"></script>
<script src="https://cdn.datatables.net/1.10.15/js/dataTables.bootstrap4.min.js "></script>  

<script>
    $('#tbl1').dataTable({});
</script>

StudentController.cs:

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.EntityFrameworkCore;
using StudentApp.Models.Entity;
using System.Linq;

namespace StudentApp.Controllers
{
    public class StudentTableController : Controller
    {
        StudentDatabaseContext db = new StudentDatabaseContext();
        public IActionResult Index(string p)
        {
            var degerler = from d in db.StudentTables select d;
            if (!string.IsNullOrEmpty(p))
            {
                degerler = degerler.Where(m => m.Name.Contains(p));
            }
            return View(degerler.ToList());
        }
        [HttpGet]
        public ActionResult AddStudent()
        {
            List<SelectListItem> GetClass = new List<SelectListItem>();
            foreach (var item in db.ClassTables.ToList())
            {
                GetClass.Add(new SelectListItem { Text = item.ClassName, Value = item.Id.ToString() });
            }
            ViewBag.ClassList = GetClass;
            return View(new StudentTable());
        }
        [HttpPost]
        public ActionResult AddStudent(StudentTable st)
        {
            db.StudentTables.Add(st);
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        public IActionResult Delete(int id)
        {
            var student = db.StudentTables.SingleOrDefault(i => i.Id == id);
            db.StudentTables.Remove(student);
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        [HttpGet]
        public ActionResult EditStudent(int id)
        {
            var info = db.StudentTables.SingleOrDefault(i => i.Id == id);
            List<SelectListItem> GetClass = new List<SelectListItem>();
            foreach (var item in db.ClassTables.ToList())
            {
                GetClass.Add(new SelectListItem { Text = item.ClassName, Value = item.Id.ToString() });
            }
            ViewBag.ClassT = GetClass;
            return View("EditStudent", info);

        }
        [HttpPost]
        public ActionResult EditStudent(StudentTable p)
        {
            var StudentT = db.StudentTables.SingleOrDefault(i => i.Id == p.Id);
            StudentT.Name = p.Name;
            StudentT.ClassId = p.ClassId;
            db.SaveChanges();
            return RedirectToAction("Index");
        }
    }
}

StudentTable.cs:

using System;
using System.Collections.Generic;

namespace StudentApp.Models.Entity
{
    public partial class StudentTable
    {
        public int Id { get; set; }
        public string? Name { get; set; }
        public int ClassId { get; set; }

        public virtual ClassTable? Class { get; set; }
    }
}

When I run the codes, I get the following error on the Index.cshtml page:

Why could this be? How can I fix the error? Thanks in advance for your help.

CodePudding user response：

You need an eager loading for the StudentTable entities to load its related Class entities.

public IActionResult Index(string p)
{
    ...

    return View(degerler
        .Include(x => x.Class)
        .ToList());
}

While Class property is nullable type in StudentTable, use ?. null-conditional operator to prevent accessing to inner property when the parent is null.

View

@item.Class?.ClassName