c# - Entity Framework 的种子方法错误，未在代码优先模型中创建表

Question

我在 EF 中使用 Code First 方法。当我尝试从包管理器控制台更新数据库时，出现以下错误。

Running Seed method.
System.ArgumentNullException: Value cannot be null.
Parameter name: set
   at System.Data.Entity.Utilities.Check.NotNull[T](T value, String parameterName)
   at System.Data.Entity.Migrations.DbSetMigrationsExtensions.AddOrUpdate[TEntity](IDbSet`1 set, Expression`1 identifierExpression, TEntity[] entities)
   at AKClasses.Migrations.Configuration.Seed(AKClassDb context) in c:\Users\amol.kshirsagar\Documents\AmolKshirsagar\RnD\MVC\MVC5\AKClasses\AKClasses\Migrations\Configuration.cs:line 19
   at System.Data.Entity.Migrations.DbMigrationsConfiguration`1.OnSeed(DbContext context)
   at System.Data.Entity.Migrations.DbMigrator.SeedDatabase()
   at System.Data.Entity.Migrations.Infrastructure.MigratorLoggingDecorator.SeedDatabase()
   at System.Data.Entity.Migrations.DbMigrator.Upgrade(IEnumerable`1 pendingMigrations, String targetMigrationId, String lastMigrationId)
   at System.Data.Entity.Migrations.Infrastructure.MigratorLoggingDecorator.Upgrade(IEnumerable`1 pendingMigrations, String targetMigrationId, String lastMigrationId)
   at System.Data.Entity.Migrations.DbMigrator.UpdateInternal(String targetMigration)
   at System.Data.Entity.Migrations.DbMigrator.<>c__DisplayClassc.<Update>b__b()
   at System.Data.Entity.Migrations.DbMigrator.EnsureDatabaseExists(Action mustSucceedToKeepDatabase)
   at System.Data.Entity.Migrations.Infrastructure.MigratorBase.EnsureDatabaseExists(Action mustSucceedToKeepDatabase)
   at System.Data.Entity.Migrations.DbMigrator.Update(String targetMigration)
   at System.Data.Entity.Migrations.Infrastructure.MigratorBase.Update(String targetMigration)
   at System.Data.Entity.Migrations.Design.ToolingFacade.UpdateRunner.Run()
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.AppDomain.DoCallBack(CrossAppDomainDelegate callBackDelegate)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Run(BaseRunner runner)
   at System.Data.Entity.Migrations.Design.ToolingFacade.Update(String targetMigration, Boolean force)
   at System.Data.Entity.Migrations.UpdateDatabaseCommand.<>c__DisplayClass2.<.ctor>b__0()
   at System.Data.Entity.Migrations.MigrationsDomainCommand.Execute(Action command)
Value cannot be null.
Parameter name: set

以下是我的模型类

我的数据库上下文类

using System.Data.Entity;

namespace AKClasses.Models
{
    public class AKClassDb : DbContext 
    {
        public DbSet<Student> Students;
        public DbSet<Subject> Subjects;
    }
}

我的学生类(class)

namespace AKClasses.Models
{
    public class Student
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }

    }
}

我的学科类

using System.Collections.Generic;
namespace AKClasses.Models
{
    public class Subject
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public virtual List<Student> Students { get; set; }
    }
}

编辑:重要的 My Configuration.cs 类

namespace AKClasses.Migrations
{
    using AKClasses.Models;
    using System.Data.Entity.Migrations;

    public sealed class Configuration : DbMigrationsConfiguration<AKClasses.Models.AKClassDb>
    {
        public Configuration()
        {
            AutomaticMigrationsEnabled = true;
        }

        protected override void Seed(AKClasses.Models.AKClassDb context)
        {

            context.Subjects.AddOrUpdate(c => c.Name, new Subject { Name = "Marathi" }, new Subject { Name = "English" });
        }
    }
}

不知何故，我能够在我的 SSMS 中看到已创建的数据库，但看不到表格。

Answer 1

DbSet<T>应该是属性(property)，而不是公共(public)领域。

尝试将它们更改为

public DbSet<Student> Students { get; set; }
public DbSet<Subject> Subjects { get; set; }