System.InvalidOperationException: The type MyContext+Northwind was not mapped.

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Data.Entity;
 6 using System.ComponentModel.DataAnnotations;
 7 
 8 namespace TestCodeFirst
 9 {
10     public class MyContext
11     {
12         public class Product
13         {
14             public int ProductID { get; set; }
15             public int CategoryID { get; set; }
16             [Required(ErrorMessage = "Product Name不能为空")]
17             public string ProductName { get; set; }
18             public Decimal? UnitPrice { get; set; }
19             public bool Discontinued { get; set; }
20             public virtual Category Category { get; set; }
21         }
22 
23         public class Category
24         {
25             public int CategoryID { get; set; }
26             public string CategoryName { get; set; }
27             public string Description { get; set; }
28             public byte[] Picture { get; set; }
29             public virtual ICollection<Product> Product { get; set; }
30         }
31         public class Northwind : DbContext
32         {
33             public DbSet<Product> Products { get; set; }
34             public DbSet<Category> Categorys { get; set; }
35         }
36     }
37 }

 1 namespace TestCodeFirst
 2 {
 3        public class Product
 4        {
 5            public int ProductID { get; set; }
 6            public int CategoryID { get; set; }
 7              [Required(ErrorMessage="Product Name不能为空")]
 8            public string ProductName { get; set; }
 9            public Decimal? UnitPrice { get; set; }
10            public bool Discontinued { get; set; }
11            public virtual Category Category { get; set; }
12        }
13 
14        public class Category
15        {
16            public int CategoryID { get; set; }
17            public string CategoryName { get; set; }
18            public string Description { get; set; }
19            public byte[] Picture { get; set; }
20            public virtual ICollection<Product> Product { get; set; }
21        }
22        public class Northwind:DbContext
23        {
24            public DbSet<Product> Products { get; set; }
25            public DbSet<Category> Categorys { get; set; }
26     }
27 }