实体框架:在包管理器控制台VS 2012中添加迁移时引发Null引用异常

本文关键字：迁移 添加 异常 引用 Null 2012 框架 管理器 VS 控制台 实体 | 更新日期: 2023-09-27 18:26:01

我一直在我的mvc项目中使用实体框架代码优先迁移。但最近，在修改了我的模型类并添加了新的模型类之后，当我尝试在包管理器控制台中使用添加迁移时，我不断收到这样的错误："Object reference not set to a instance of a Object"我正在使用entity framework version 5，当我试着使用版本6时，它看不到我的DbContext，这是我的堆栈跟踪，请欢迎所有建议。

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Data.Entity.ModelConfiguration.Configuration.Types.EntityTypeConfiguration.Configure(EdmEntityType entityType, EdmModel model)
   at System.Data.Entity.ModelConfiguration.Configuration.ModelConfiguration.ConfigureEntities(EdmModel model)
   at System.Data.Entity.DbModelBuilder.Build(DbProviderManifest providerManifest, DbProviderInfo providerInfo)
   at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
   at System.Data.Entity.Infrastructure.EdmxWriter.WriteEdmx(DbContext context, XmlWriter writer)
   at System.Data.Entity.Migrations.Extensions.DbContextExtensions.<>c__DisplayClass1.<GetModel>b__0(XmlWriter w)
   at System.Data.Entity.Migrations.Extensions.DbContextExtensions.GetModel(Action`1 writeXml)
   at System.Data.Entity.Migrations.Extensions.DbContextExtensions.GetModel(DbContext context)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration, DbContext usersContext)
   at System.Data.Entity.Migrations.DbMigrator..ctor(DbMigrationsConfiguration configuration)
   at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.GetMigrator()
   at System.Data.Entity.Migrations.Design.ToolingFacade.GetPendingMigrationsRunner.RunCore()
   at System.Data.Entity.Migrations.Design.ToolingFacade.BaseRunner.Run()
Object reference not set to an instance of an object.

实体框架:在包管理器控制台VS 2012中添加迁移时引发Null引用异常

您的repo中缺少一个部分，但我发现其他人在这里报告了相同/类似的问题，所以我将使用他们的示例。这是复印件：

public class Person
{
    public int PersonID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public virtual ICollection<Meeting> Meeting { get; set; }
}
public class Meeting
{
    public int MeetingID { get; set; }
    [ForeignKey("Customer")]
    public int CustomerID { get; set; }
    public virtual Person Customer { get; set; }
    [ForeignKey("SalesAgent")]
    public int SalesAgentID { get; set; }
    public virtual Person SalesAgent { get; set; }
}
public class MyContext : DbContext
{
    public DbSet<Person> People { get; set; }
    public DbSet<Meeting> Meetings { get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Meeting>().HasRequired(m => m.Customer).WithMany(p => p.Meeting);
        modelBuilder.Entity<Meeting>().HasRequired(m => m.SalesAgent).WithMany(p => p.Meeting);
    }
}
class Program
{
    static void Main(string[] args)
    {
        using (var ctx = new MyContext())
        {
            EdmxWriter.WriteEdmx(ctx, XmlWriter.Create(Console.Out, new XmlWriterSettings { Indent = true }));
        }
    }
}

EF5中存在一个导致NullReferenceException的错误。由于另一个错误，这在EF6中也不起作用，但我相信这些都与此无关。我认为这里的意图是将Person实体的一个导航属性映射到Meeting实体的两个导航属性，EF不支持这一点（类似地，您无法仅通过查看Person.Meeting集合的内容来判断用户是作为客户还是作为销售代理（或两者兼有？）-您必须比较密钥值/引用-EF只是不起作用）。为了解决这个问题，模型需要更改如下：

public class Person
{
    public int PersonID { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public virtual ICollection<Meeting> MeetingsAsCustomer { get; set; }
    public virtual ICollection<Meeting> MeetingAsSalesAgent { get; set; }
}
public class Meeting
{
    public int MeetingID { get; set; }
    [ForeignKey("Customer")]
    public int CustomerID { get; set; }
    public virtual Person Customer { get; set; }
    [ForeignKey("SalesAgent")]
    public int SalesAgentID { get; set; }
    public virtual Person SalesAgent { get; set; }
}
public class MyContext : DbContext
{
    public DbSet<Person> People { get; set; }
    public DbSet<Meeting> Meetings { get; set; }
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Meeting>().HasRequired(m => m.Customer).WithMany(p => p.MeetingsAsCustomer);
        modelBuilder.Entity<Meeting>().HasRequired(m => m.SalesAgent).WithMany(p => p.MeetingAsSalesAgent);
    }

现在，我们在Person属性上有两个导航属性，它们映射到Meeting实体上相应的导航属性。

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