實體框架的種子方法錯誤，未在代碼優先模型中創建表

Question

我在EF中使用“代碼優先”方法。 當我嘗試從Package Manager控制台更新數據庫時，出現以下錯誤。

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

以下是我的模特班

我的數據庫上下文類

using System.Data.Entity;

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

我的學生班

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

    }
}

我的科目

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; }
    }
}

編輯：重要我的Configuration.cs類

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" });
        }
    }
}

我能以某種方式看到在SSMS中創建的數據庫，但是看不到表格。

Answer 1

DbSet<T>應該是屬性，而不是公共字段。

嘗試將它們更改為

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