Base enity from NHibernate Cookbook

Entity.cs

/// <summary>
    /// Base Entity for NHibernate that implements equality
    /// </summary>
    /// <typeparam name="TId">Type of the identifier of the entity</typeparam>
    /// <remarks>Code from the book NHibernate 3.0 Cookbook - Chapter 1 - Setting up a base entity class</remarks>
    public abstract class Entity<TId>
    {
        /// <summary>
        /// Gets or sets the id
        /// </summary>
        /// <value>
        /// The id
        /// </value>
        public virtual TId Id { get; protected set; }

        /// <summary>
        /// Gets or sets the version. Used for optimistic concurrency
        /// </summary>
        /// <value>
        /// The version
        /// </value>
        public virtual int Version { get; set; }

        /// <summary>
        /// Implements equality, needed for NHibernate
        /// </summary>
        public override bool Equals(object obj)
        {
            return Equals(obj as Entity<TId>);
        }

        private static bool IsTransient(Entity<TId> obj)
        {
            return obj != null &&
                   Equals(obj.Id, default(TId));
        }

        private Type GetUnproxiedType()
        {
            return GetType();
        }

        /// <summary>
        /// Implements equality, needed for NHibernate
        /// </summary>
        public virtual bool Equals(Entity<TId> other)
        {
            if (other == null)
                return false;

            if (ReferenceEquals(this, other))
                return true;

            if (!IsTransient(this) &&
                !IsTransient(other) &&
                Equals(Id, other.Id))
            {
                var otherType = other.GetUnproxiedType();
                var thisType = GetUnproxiedType();
                return thisType.IsAssignableFrom(otherType) ||
                       otherType.IsAssignableFrom(thisType);
            }

            return false;
        }

        private int? _hashCode;

        /// <summary>
        /// Implements equality, needed for NHibernate
        /// </summary>
        public override int GetHashCode()
        {
            if (_hashCode.HasValue)
            {
                return _hashCode.Value;
            }

            var isTransient = Equals(Id, default(TId));

            if (isTransient)
            {
                _hashCode = base.GetHashCode();
                return _hashCode.Value;
            }

            return Id.GetHashCode();
        }
    }