实体框架:在包管理器控制台VS 2012中添加迁移时引发的空引用异常
实体框架:在包管理器控制台VS 2012中添加迁移时引发的空引用异常
提问于 2013-08-03 20:17:19
我一直在我的mvc项目中使用实体框架代码第一次迁移。但是最近在修改了我的模型类并添加了新的类之后，当我尝试在包管理器控制台中使用添加迁移时，我一直收到这样的错误：“对象引用没有设置为一个对象的实例”，我使用的是entity framework version 5，当我尝试使用version 6但它不会看到我的DbContext时，这里是我的堆栈跟踪，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.回答 1
Stack Overflow用户
发布于 2013-09-12 23:17:20
您的repro中缺少一段内容，但我找到了其他人报告了同样的/类似的问题这里，所以我将使用他们的例子。这是复制品：
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中有一个导致NullReferenceException的错误。这在EF6中也不起作用，因为有一个不同的bug，但我认为这一切在这里都没有实际意义。我认为这里的意图是将Person实体中的一个导航属性映射到来自会议实体的两个导航属性，而EF不支持这一点(类似地，您不能通过查看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(或者两者都是？)-您必须比较关键值/引用- EF只是不这样做)。要解决这一问题，需要对模型进行如下修改：
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，Person属性上有两个导航属性，它们被映射到会议实体上的相应导航属性。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/18036902
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