entity object cannot be referenced by multiple ins

问题:

I am trying to save Employee details, which has references with City. But everytime I try to save my contact, which is validated I get the exception \"ADO.Net Entity Framework An entity object cannot be referenced by multiple instances of IEntityChangeTracker\"

I had read so many post but still not getting the exact idea of what to do... my Save button click code is given below

protected void Button1_Click(object sender, EventArgs e)
    {
        EmployeeService es = new EmployeeService();
        CityService cs = new CityService();

        DateTime dt = new DateTime(2008, 12, 12);
        Payroll.Entities.Employee e1 = new Payroll.Entities.Employee();

        Payroll.Entities.City city1 = cs.SelectCity(Convert.ToInt64(cmbCity.SelectedItem.Value));

        e1.Name = \"Archana\";
        e1.Title = \"aaaa\";
        e1.BirthDate = dt;
        e1.Gender = \"F\";
        e1.HireDate = dt;
        e1.MaritalStatus = \"M\";
        e1.City = city1;        

        es.AddEmpoyee(e1,city1);
    }

and Employeeservice Code

public string AddEmpoyee(Payroll.Entities.Employee e1, Payroll.Entities.City c1)
        {
            Payroll_DAO1 payrollDAO = new Payroll_DAO1();
            payrollDAO.AddToEmployee(e1);  //Here I am getting Error..
            payrollDAO.SaveChanges();
            return \"SUCCESS\";
        }

回答1:

Because these two lines ...

EmployeeService es = new EmployeeService();
CityService cs = new CityService();

... don\'t take a parameter in the constructor, I guess that you create a context within the classes. When you load the city1...

Payroll.Entities.City city1 = cs.SelectCity(...);

...you attach the city1 to the context in CityService. Later you add a city1 as a reference to the new Employee e1 and add e1 including this reference to city1 to the context in EmployeeService. As a result you have city1 attached to two different context which is what the exception complains about.

You can fix this by creating a context outside of the service classes and injecting and using it in both services:

EmployeeService es = new EmployeeService(context);
CityService cs = new CityService(context); // same context instance

Your service classes look a bit like repositories which are responsible for only a single entity type. In such a case you will always have trouble as soon as relationships between entities are involved when you use separate contexts for the services.

You can also create a single service which is responsible for a set of closely related entities, like an EmployeeCityService (which has a single context) and delegate the whole operation in your Button1_Click method to a method of this service.

回答2:

Steps to reproduce can be simplified to this:

var contextOne = new EntityContext();
var contextTwo = new EntityContext();

var user = contexOne.Users.FirstOrDefault();

var group = new Group();
group.User = user;

contextTwo.Groups.Add(group);
contextTwo.SaveChanges();

Code without error:

var context = new EntityContext();

var user = context.Users.FirstOrDefault();

var group = new Group();
group.User = user; // Be careful when you set entity properties. 
// Be sure that all objects came from the same context

context.Groups.Add(group);
context.SaveChanges();

回答3:

This is an old thread, but another solution, which I prefer, is just update the cityId and not assign the hole model City to Employee... to do that Employee should look like:

public class Employee{
...
public int? CityId; //The ? is for allow City nullable
public virtual City City;
}

Then it\'s enough assigning:

e1.CityId=city1.ID;

回答4:

I had the same problem but my issue with the @Slauma\'s solution (although great in certain instances) is that it recommends that I pass the context into the service which implies that the context is available from my controller. It also forces tight coupling between my controller and service layers.

I\'m using Dependency Injection to inject the service/repository layers into the controller and as such do not have access to the context from the controller.

My solution was to have the service/repository layers use the same instance of the context - Singleton.

Context Singleton Class:

Reference: http://msdn.microsoft.com/en-us/library/ff650316.aspx
and http://csharpindepth.com/Articles/General/Singleton.aspx

public sealed class MyModelDbContextSingleton
{
  private static readonly MyModelDbContext instance = new MyModelDbContext();

  static MyModelDbContextSingleton() { }

  private MyModelDbContextSingleton() { }

  public static MyModelDbContext Instance
  {
    get
    {
      return instance;
    }
  }
}  

Repository Class:

public class ProjectRepository : IProjectRepository
{
  MyModelDbContext context = MyModelDbContextSingleton.Instance;

Other solutions do exist such as instantiating the context once and passing it into the constructors of your service/repository layers or another I read about which is implementing the Unit of Work pattern. I\'m sure there are more...

回答5:

Alternatively to injection and even worse Singleton, you can call Detach method before Add.

EntityFramework 6: ((IObjectContextAdapter)cs).ObjectContext.Detach(city1);

EntityFramework 4: cs.Detach(city1);

There is yet another way, in case you don\'t need first DBContext object. Just wrap it with using keyword:

Payroll.Entities.City city1;
using (CityService cs = new CityService())
{
  city1 = cs.SelectCity(Convert.ToInt64(cmbCity.SelectedItem.Value));
}

回答6:

In my case, I was using the ASP.NET Identity Framework. I had used the built in UserManager.FindByNameAsync method to retrieve an ApplicationUser entity. I then tried to reference this entity on a newly created entity on a different DbContext. This resulted in the exception you originally saw.

I solved this by creating a new ApplicationUser entity with only the Id from the UserManager method and referencing that new entity.

回答7:

I had the same problem and I could solve making a new instance of the object that I was trying to Update. Then I passed that object to my reposotory.

回答8:

In this case, it turns out the error is very clear: Entity Framework cannot track an entity using multiple instances of IEntityChangeTracker or typically, multiple instances of DbContext. The solutions are: use one instance of DbContext; access all needed entities through a single repository (depending on one instance of DbContext); or turning off tracking for all entities accessed via a repository other than the one throwing this particular exception.

When following an inversion of control pattern in .Net Core Web API, I frequently find that I have controllers with dependencies such as:

private readonly IMyEntityRepository myEntityRepo; // depends on MyDbContext
private readonly IFooRepository fooRepo; // depends on MyDbContext
private readonly IBarRepository barRepo; // depends on MyDbContext
public MyController(
    IMyEntityRepository myEntityRepo, 
    IFooRepository fooRepo, 
    IBarRepository barRepo)
{
    this.fooRepo = fooRepo;
    this.barRepo = barRepo;
    this.myEntityRepo = myEntityRepo;
}

and usage like

...
myEntity.Foo = await this.fooRepository.GetFoos().SingleOrDefaultAsync(f => f.Id == model.FooId);
if (model.BarId.HasValue)
{
    myEntity.Foo.Bar = await this.barRepository.GetBars().SingleOrDefaultAsync(b => b.Id == model.BarId.Value);
}

...
await this.myEntityRepo.UpdateAsync(myEntity); // this throws an error!

Since all three repositories depend on different DbContext instances per request, I have two options to avoid the problem and maintain separate repositories: change the injection of the DbContext to create a new instance only once per call:

// services.AddTransient<DbContext, MyDbContext>(); <- one instance per ctor. bad
services.AddScoped<DbContext, MyDbContext>(); // <- one instance per call. good!

or, if the child entity is being used in a read-only manner, turning off tracking on that instance:

myEntity.Foo.Bar = await this.barRepo.GetBars().AsNoTracking().SingleOrDefault(b => b.Id == model.BarId);

回答9:

Use the same DBContext object throughout the transaction.

回答10:

Error source:

ApplicationUser user = await UserManager.FindByIdAsync(User.Identity.Name);
ApplicationDbContext db = new ApplicationDbContent();
db.Users.Uploads.Add(new MyUpload{FileName=\"newfile.png\"});
await db.SavechangesAsync();/ZZZZZZZ

Hope someone saves some precious time