Why can't I override GetHashCode on a many-to-many entity in EF4?

Question

I have a many-to-many relationship in my Entity Framework 4 model (which works with a MS SQL Server Express): Patient-PatientDevice-Device. I'm using Poco, so my PatientDevice-class looks like this:

public class PatientDevice
{
    protected virtual Int32 Id { get; set; }
    protected virtual Int32 PatientId { get; set; }
    public virtual Int32 PhysicalDeviceId { get; set; }
    public virtual Patient Patient { get; set; }
    public virtual Device Device { get; set; }

    //public override int GetHashCode()
    //{
    //    return Id;
    //}
}

All works well when I do this:

var context = new Entities();
var patient = new Patient();
var device = new Device();

context.PatientDevices.AddObject(new PatientDevice { Patient = patient, Device = device });
context.SaveChanges();

Assert.AreEqual(1, patient.PatientDevices.Count);

foreach (var pd in context.PatientDevices.ToList())
{
    context.PatientDevices.DeleteObject(pd);
}
context.SaveChanges();

Assert.AreEqual(0, patient.PatientDevices.Count);

But if I uncomment GetHashCode in PatientDevice-class, the patient still has the PatientDevice added earlier.

What is wrong in overriding GetHashCode and returning the Id?

What happens when you create a new `context` for the add and the delete action? — Pieter van Ginkel, Nov 15 '10 at 14:58
Deleting with a new context works (I forgot to mention: the PatientDevice is deleted on the db with just one context also). — Papa Mufflon, Nov 15 '10 at 15:09
What is the value of `Id` of `patient.PatientDevides[0]` at the moment of your `Assert` (at the every end) with the `GetHashCode()` implemented? — Pieter van Ginkel, Nov 15 '10 at 16:04
Are you overriding `GetHashCode` but not `Equals`? `GetHashCode` has nothing to do with object identity. — Gabe, Nov 15 '10 at 16:53

Pieter van Ginkel · Accepted Answer · 2010-11-15T16:11:26.343

The reason may very well be that the class type is not part of the hash code, and that the entity framework has difficulty distinguishing between the different types.

Try the following:

public override int GetHashCode()
{
    return Id ^ GetType().GetHashCode();
}

Another problem is that the result of GetHashCode() may not change during the lifetime of an object under certain circumstances, and these may apply for the entity framework. This together with the Id begin 0 when it's created also poses problems.

An alternative of GetHashCode() is:

private int? _hashCode;

public override int GetHashCode()
{
    if (!_hashCode.HasValue)
    {
        if (Id == 0)
            _hashCode.Value = base.GetHashCode();
        else
            _hashCode.Value = Id;
            // Or this when the above does not work.
            // _hashCode.Value = Id ^ GetType().GetHashCode();
    }

    return _hasCode.Value;
}

Taken from http://nhforge.org/blogs/nhibernate/archive/2008/09/06/identity-field-equality-and-hash-code.aspx.

Remembered an old post which tackles transient entities (`Id == 0`) and the fact that the result of `GetHashCode()` should not change during the lifetime of an entity. Give it a try. — Pieter van Ginkel, Nov 15 '10 at 16:12

Why can't I override GetHashCode on a many-to-many entity in EF4?

1 Answers1

Linked