[c#] EntityType has no key defined error

Controller:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using MvcApplication1.Models;
using System.ComponentModel.DataAnnotations.Schema;

namespace MvcApplication1.Controllers
{
    public class studentsController : Controller
    {
        //
        // GET: /students/

        public ActionResult details()
        {
            int id = 16;
            studentContext std = new studentContext();
           student first = std.details.Single(m => m.RollNo == id);
            return View(first);
        }

    }
}

DbContext Model:

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

namespace MvcApplication1.Models
{
    public class studentContext : DbContext
    {
        public DbSet<student> details { get; set; }
    }
}

Model:

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

namespace MvcApplication1.Models
{
    [Table("studentdetails")]
    public class student
    {
        public int RollNo;
        public string Name;
        public string Stream;
        public string Div;
    }
}

Database table:

CREATE TABLE [dbo].[studentdetails](
    [RollNo] [int] NULL,
    [Name] [nvarchar](50) NULL,
    [Stream] [nvarchar](50) NULL,
    [Div] [nvarchar](50) NULL
)  

In global.asax.cs

Database.SetInitializer<MvcApplication1.Models.studentContext>(null);

The above code lists all the classes I am working on. Upon running my application am receiving the error:

"One or more validation errors were detected during model generation" along with "Entity type has no key defined".

The object must contain a field which will be used as the Primary Key, if you have a field named Id then this will by default be the primary key of the object to which Entity Framework will link to.

Otherwise, you should add the [Key] attribute above that field you wanna use as the Primary Key, and you'll also need to add the namespace System.ComponentModel.DataAnnotations:

public class myClass
{
    public int Id { get; set; }
    [Key]
    public string Name { get; set; }
}

https://stackoverflow.com/a/51180941/7003760

1. Make sure public members of student class are defined as properties w/ {get; set;} (yours are public variables - a common mistake).

2. Place a [Key] annotation on top of your chosen property.

There are several reasons this can happen. Some of these I found here, others I discovered on my own.

• If the property is named something other than Id, you need to add the [Key] attribute to it.
• The key needs to be a property, not a field.
• The key needs to be public
• The key needs to be a CLS-compliant type, meaning unsigned types like uint, ulong etc. are not allowed.
• This error can also be caused by configuration mistakes.

For me, the model was fine. It was because I had 2 different versions of entity framework. One version for the web project and another one for the common project which contains the models.

I just had to consolidate the nuget packages.

Enjoy!

i know that this post is late but this solution helped me:

    [Key]
    [Column(Order = 0)]
    public int RoleId { get; set; }

added [Column(Order = 0)] after [Key] can be added by increment by 1:

    [Key]
    [Column(Order = 1)]
    public int RoleIdName { get; set; }

etc...

I too solved this issue in my own project by solving this particular line in my code. I added the following.

[DatabaseGenerated(DatabaseGeneratedOption.None)]

After realizing my mistake I then went and changed it to

[DatabaseGenerated(DatabaseGeneratedOption.Identity)]

This further ensures that the field named "Id" increments in value each time a new row is inserted in the database

You don't have to use key attribute all the time. Make sure the mapping file properly addressed the key

this.HasKey(t => t.Key);
this.ToTable("PacketHistory");
this.Property(p => p.Key)
            .HasColumnName("PacketHistorySK");

and don't forget to add the mapping in the Repository's OnModelCreating

modelBuilder.Configurations.Add(new PacketHistoryMap());

Using the [key] didn't work for me but using an id property does it. I just add this property in my class.

public int id {get; set;}

The reason why EF framework prompt 'no key' is that EF framework needs a primary key in database. To declaratively tell EF which property the key is, you add a [Key] annotation. Or, the quickest way, add an ID property. Then, the EF framework will take ID as the primary key by default.

Additionally Remember, Don't forget to add public keyword like this

[Key] 
int RoleId { get; set; } //wrong method

you must use Public keyword like this

[Key] 
public int RoleId { get; set; } //correct method

All is right but in my case i have two class like this

namespace WebAPI.Model
{
   public class ProductsModel
{ 
        [Table("products")]
        public class Products
        {
            [Key]
            public int slno { get; set; }

            public int productId { get; set; }
            public string ProductName { get; set; }

            public int Price { get; set; }
}
        }
    }

After deleting the upper class it works fine for me.

In my case, I was getting the error when creating an "MVC 5 Controller with view, using Entity Framework".

I just needed to Build the project after creating the Model class and didn't need to use the [Key] annotation.