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

System.InvalidOperationException: The type MyContext+Northwind was not mapped. Check that the type has not been explicitly excluded by using the Ignore method or NotMappedAttribute data annotation. Verify that the type was defined as a class, is not primitive, nested or generic, and does not inherit from EntityObject.

错误原因：

 entity framework不能将内部类与数据库匹配。

错误代码如下所示:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Entity;
using System.ComponentModel.DataAnnotations;

namespace TestCodeFirst
{
    public class MyContext
    {
        public class Product
        {
            public int ProductID { get; set; }
            public int CategoryID { get; set; }
            [Required(ErrorMessage = "Product Name不能为空")]
            public string ProductName { get; set; }
            public Decimal? UnitPrice { get; set; }
            public bool Discontinued { get; set; }
            public virtual Category Category { get; set; }
        }

        public class Category
        {
            public int CategoryID { get; set; }
            public string CategoryName { get; set; }
            public string Description { get; set; }
            public byte[] Picture { get; set; }
            public virtual ICollection<Product> Product { get; set; }
        }
        public class Northwind : DbContext
        {
            public DbSet<Product> Products { get; set; }
            public DbSet<Category> Categorys { get; set; }
        }
    }
}

解决的办法：Northwind作为内部类。正确的代码如下所示

namespace TestCodeFirst
{
       public class Product
       {
           public int ProductID { get; set; }
           public int CategoryID { get; set; }
             [Required(ErrorMessage="Product Name不能为空")]
           public string ProductName { get; set; }
           public Decimal? UnitPrice { get; set; }
           public bool Discontinued { get; set; }
           public virtual Category Category { get; set; }
       }

       public class Category
       {
           public int CategoryID { get; set; }
           public string CategoryName { get; set; }
           public string Description { get; set; }
           public byte[] Picture { get; set; }
           public virtual ICollection<Product> Product { get; set; }
       }
       public class Northwind:DbContext
       {
           public DbSet<Product> Products { get; set; }
           public DbSet<Category> Categorys { get; set; }
    }
}