entity object cannot be referenced by multiple ins

Question

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";
        }

Answer 1

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.

Answer 2

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);

Answer 3

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));
}

Answer 4

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();

Answer 5

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.

Answer 6

Use the same DBContext object throughout the transaction.