Does this solve Nhibernate identity problem and GetHashCode issues?

Question

The solution I propose involves quite a bit of code, but you can just copy it all and past it in a VS test solution assuming you have SqLite installed, and you should be able to

Answer 1

That's an interesting approach but instead of taking the time to understand and critique I will just offer my solution to this problem.

I don't like the idea of a generic entity base class, so my solution only supports int, Guid and string identities. Some of the code below, such as using a Func to get the hash code, only exists to support case-insensitive string comparisons. If I ignored string identifiers (and I wish I could), the code would be more compact.

This code passes the unit tests I have for it and hasn't let me down in our applications but I'm sure there are edge cases. The only one I've thought of is: If I new and save an entity it will keep its original hash code, but if after the save I retrieve an instance of the same entity from the database in another session it will have a different hash code.

Feedback welcome.

Base class:

[Serializable]
public abstract class Entity
{
    protected int? _cachedHashCode;

    public abstract bool IsTransient { get; }

    // Check equality by comparing transient state or id.
    protected bool EntityEquals(Entity other, Func idEquals)
    {
        if (other == null)
        {
            return false;
        }
        if (IsTransient ^ other.IsTransient)
        {
            return false;
        }
        if (IsTransient && other.IsTransient)
        {
            return ReferenceEquals(this, other);
        }
        return idEquals.Invoke();
    }

    // Use cached hash code to ensure that hash code does not change when id is assigned.
    protected int GetHashCode(Func idHashCode)
    {
        if (!_cachedHashCode.HasValue)
        {
            _cachedHashCode = IsTransient ? base.GetHashCode() : idHashCode.Invoke();
        }
        return _cachedHashCode.Value;
    }
}

int identity:

[Serializable]
public abstract class EntityIdentifiedByInt : Entity
{
    public abstract int Id { get; }

    public override bool IsTransient
    {
        get { return Id == 0; }
    }

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

    public virtual bool Equals(EntityIdentifiedByInt other)
    {
        return EntityEquals(other, () => Id == other.Id);
    }

    public override int GetHashCode()
    {
        return GetHashCode(() => Id);
    }
}

Guid identity:

[Serializable]
public abstract class EntityIdentifiedByGuid : Entity
{
    public abstract Guid Id { get; }

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

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

    public virtual bool Equals(EntityIdentifiedByGuid other)
    {
        return EntityEquals(other, () => Id == other.Id);
    }

    public override int GetHashCode()
    {
        return GetHashCode(() => Id.GetHashCode());
    }
}

string identity:

[Serializable]
public abstract class EntityIdentifiedByString : Entity
{
    public abstract string Id { get; }

    public override bool IsTransient
    {
        get { return Id == null; }
    }

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

    public virtual bool Equals(EntityIdentifiedByString other)
    {
        Func idEquals = () => string.Equals(Id, other.Id, StringComparison.OrdinalIgnoreCase);
        return EntityEquals(other, idEquals);
    }

    public override int GetHashCode()
    {
        return GetHashCode(() => Id.ToUpperInvariant().GetHashCode());
    }
}