Mvc无效的操作异常

Question

I need to retrieve employees from my-sql server database depending on the id parameter passed. For example: http://localhost:66666/employee/details/1 <---the parameter and match the parameter with an id in the database and display it in the view. For some reason I am getting an invalid operation exception on this line in the employee controller: Employee employee = EmployeeContext.Employees.Single(emp => emp.EmployeeID == id);

I am not sure if the error actually originates there.

employee controller

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

namespace MVCTutorials.Controllers
{
    public class EmployeeController : Controller
    {
        // GET: Employee
        public ActionResult Details(int id)//id =paramenter 
        {

            EmployeeContext EmployeeContext = new EmployeeContext();

            Employee employee = EmployeeContext.Employees.Single(emp => emp.EmployeeID == id);
            return View(employee);

        }
    }
}

employee model

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Web;

namespace MVCTutorials.Models
{
    [Table("TblEmp")]
    public class Employee
    {
        [Key]
        public int EmployeeID { get; set; }
        public string Name { get; set; }
        public string Gender { get; set; }
        public string City { get; set; }
    }
}

employee view

@model MVCTutorials.Models.Employee
@{
    ViewBag.Title = "Employeee Details";
}

<h2>Employeee Details</h2>

<table style="font-family:Arial;">
    <tr><!--The Row-->
        <td>
                <b>Employee Id: </b>
        </td>
        <td>
            @Model.EmployeeID
        </td> 
    </tr>
    <tr>
        <td>
            Name:
        </td>
        <td>
            @Model.Name
        </td>
    </tr>
    <tr>
        <td>Gender:</td>
        <td>@Model.Gender</td>
    </tr>
    <tr>
        <td>City:</td>
        <td>@Model.City </td>
    </tr>
</table>

Employee context

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Web;

namespace MVCTutorials.Models
{
    public  class EmployeeContext : DbContext
    {
        public DbSet<Employee> Employees { get; set; }
    }
}

Global.aspx

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace MVCTutorials
{
    public class MvcApplication : System.Web.HttpApplication
    {
        //Happens when appplication first starts
        protected void Application_Start()
        {
            //Purpose used to initalize your database e.g if you dont have a database created already this will create one for you(database,tables and data will all be created)
            // we pass null saying we dont want any of this
            Database.SetInitializer<MVCTutorials.Models.EmployeeContext>(null);
            AreaRegistration.RegisterAllAreas();
            GlobalConfiguration.Configure(WebApiConfig.Register);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
        }
    }
}

Answer 1

Single method Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence. so you can use SingleOrDefault function which return null if you havnt any desired row.

Employee employee = EmployeeContext.Employees.SingleOrDefault(emp => emp.EmployeeID == id);

Answer 2

According to the MSDN for Enumerable.Single() :

InvalidOperationException The input sequence contains more than one element. -or- The input sequence is empty.

I believe you have either more than one Employee or none who matches the criteria so exception is thrown.

You should either use First() if it is possible that more than one Employee exists or use SingleOrDefault and check for null .

Answer 3

Instead of using Single method, use Find and then verify if it was found.

Employee employee = EmployeeContext.Employees.Find(id);
if (employee == null)
    // not found on database 

As per documentation , Single method will success only if there is exaclty one record matching your criteria. Not more not less than one record.