Nhibernate iesicollection contains returns false

Question

Nhibernate forces you to use an Iesi Set, instead of the net 4 ISet interface. In the following snippet of code I check whether an iesi set contains an item:

    public virtual void Remove(Substance substance)
    {
        var test = _substances.First() == substance;

        if (!_substances.Contains(substance)) return;

        _substances.Remove(substance);
        substance.SubstanceGroup = null;
    }

The variable _substances references a HashedSet. I have added the test var just to check the code as a temporary measure. I have the Equals method overridden like this:

    public override int GetHashCode()
    {
        return Equals(Id, default(TId)) ? base.GetHashCode() : Id.GetHashCode();
    }

This causes the item to return the Id (Guid) as hash. If I check in the debugger I get the following result:

test
true
_substances.Contains(substance)
false
_substances.First().GetHashCode()
-2974953
substance.GetHashCode()
-2974953

How can it be that exactly the same object is not discovered in the collection using the contains method of that collection?? I can even do this in the debugger:

_substances.Contains(_substances.First())
false

Obviously, _substances.Remove(substance) doesn't work either. After some additional research I found out that NH replaces the collection with it's own Persistent Generic set. The problem arises when this set is used. If I retrieve an item from that set and I call Contains on the same set, it always returns false. I have overridden the GetHashCode and Equals, even put return true in the Equals method.

Answer 1

There's something wrong with your Equals and GetHashCode implementation because I assure you that Iesi ISet collection works correctly. The reason it is replaced by PersistentGenericSet is that ISet is just an interface, the collection has to be replaced by a concrete type. Without more code it's hard to see where the problem is so I have pasted a better equality implementation below. One problem I can see in yours is that the hash code will change after the Id is assigned, my version handles that by caching the hash code.

public class Substance
{
    private int? _cachedHashCode;

    public Substance()
    {
        Id = Guid.Empty;
    }

    public Substance(Guid id)
    {
        Id = id;
    }

    public Guid Id { get; set; }

    public bool IsTransient
    {
        get { return Id == Guid.Empty; }
    }

    public bool Equals(Substance other)
    {
        if (IsTransient ^ other.IsTransient)
        {
            return false;
        }
        if (IsTransient && other.IsTransient)
        {
            return ReferenceEquals(this, other);
        }
        return other.Id.Equals(Id);
    }

    public override bool Equals(object obj)
    {
        if (obj == null || obj.GetType() != GetType())
        {
            return false;
        }
        var other = (Substance)obj;
        return Equals(other);
    }

    public override int GetHashCode()
    {
        if (!_cachedHashCode.HasValue)
        {
            _cachedHashCode = IsTransient ? base.GetHashCode() : Id.GetHashCode();
        }
        return _cachedHashCode.Value;
    }
}

public class Mixture
{
    public Mixture()
    {
        Substances = new HashedSet<Substance>();
    }

    public ISet<Substance> Substances { get; set; }
}

public class Tests
{
    [Test]
    public void set_contains_transient_substance()
    {
        var mixture = new Mixture();
        var s1 = new Substance();
        mixture.Substances.Add(s1);
        Assert.IsTrue(mixture.Substances.Contains(s1));
    }

    [Test]
    public void set_contains_persistent_substance()
    {
        var id = Guid.NewGuid();
        var mixture = new Mixture();

        var s1 = new Substance(id);
        mixture.Substances.Add(s1);

        var s2 = new Substance(id);
        // these were created with the same id so hash code is not cached
        // and id equality is used
        Assert.IsTrue(mixture.Substances.Contains(s2));
    }

    [Test]
    public void remove_substance()
    {
        var id = Guid.NewGuid();
        var mixture = new Mixture();

        var s1 = new Substance(id);
        mixture.Substances.Add(s1);

        var s2 = new Substance(id);
        mixture.Substances.Remove(s2);
        Assert.IsTrue(mixture.Substances.Count() == 0);
    }

    [Test]
    public void hash_code_is_cached()
    {
        var s1 = new Substance(Guid.NewGuid());
        var s2 = new Substance(Guid.NewGuid());

        var mixture = new Mixture();
        mixture.Substances.Add(s1);

        Assert.IsFalse(mixture.Substances.Contains(s2));
        // assign s1 id to s2, s2 hashcode is cached so they are not equal
        s2.Id = s1.Id;
        Assert.IsFalse(mixture.Substances.Contains(s2));
    }

}