参数字典包含不可为空类型“System.Int32”的参数“id”的 null 条目。

Question

I'm facing a problem that I couldn't solve. I get this message on my browser:

The parameters dictionary contains a null entry for parameter 'id' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult Students(Int32)' in 'FirstProject.Controllers.HomeController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.

I will give you the codes that I see related to the problem:

RouteConfig.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace FirstProject
{
    public class RouteConfig
    {
        public static void RegisterRoutes(RouteCollection routes)
        {
            routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

            routes.MapRoute(
                name: "Default",
                url: "{controller}/{action}/{id}",
                defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
            );
        }
    }
}

HomeController.cs

using FirstProject.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace FirstProject.Controllers
{
    public class HomeController : Controller
    {
        level_8_coursesEntities db = new level_8_coursesEntities();
        public ActionResult Index()
        {
            List<level_8_courses> list = db.level_8_courses.ToList();
            return View(list);
        }
        public ActionResult Numberofstudents()
        {
            List<level_8_courses> list = db.level_8_courses.ToList();
            return View(list);
        }

        public ActionResult Students(int id)
        {
            List<student> studentlist = db.students.Where(x => x.corId == id).ToList();
            return View(studentlist);
        }

Students

@using FirstProject.Models;
@model List<student>

@foreach (var item in Model)
            {
                <p>@item.sName</p>
            }

I do not have enough experience in programming, so please explain in detail what I do in each file and what I write.

Thank you very much.

Answer 1

The action method public ActionResult Students(int id) is declared with parameter id as int type, but you hit this method with non-number value.

For example, you can change the method signature, like below:

public ActionResult Students(int? id)
{
    List<student> studentlist = null;
    if (id.HasValue)
    {
        studentlist = db.students.Where(x => x.corId == id).ToList();         
    }
    else
    {
        // Set the `studentlist` to some default value when `id` doesn't defined or not a number.
        studentlist = ... 
    }
    return View(studentlist);
}