ValueObject.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace PluralsightDdd.SharedKernel
{
  /// <summary>
  /// See: https://enterprisecraftsmanship.com/posts/value-object-better-implementation/
  /// </summary>
  [Serializable]
  public abstract class ValueObject : IComparable, IComparable<ValueObject>
  {
    private int? _cachedHashCode;

    protected abstract IEnumerable<object> GetEqualityComponents();

    public override bool Equals(object obj)
    {
      if (obj == null)
        return false;

      if (GetUnproxiedType(this) != GetUnproxiedType(obj))
        return false;

      var valueObject = (ValueObject)obj;

      return GetEqualityComponents().SequenceEqual(valueObject.GetEqualityComponents());
    }

    public override int GetHashCode()
    {
      if (!_cachedHashCode.HasValue)
      {
        _cachedHashCode = GetEqualityComponents()
            .Aggregate(1, (current, obj) =>
            {
              unchecked
              {
                return current * 23 + (obj?.GetHashCode() ?? 0);
              }
            });
      }

      return _cachedHashCode.Value;
    }

    public int CompareTo(object obj)
    {
      Type thisType = GetUnproxiedType(this);
      Type otherType = GetUnproxiedType(obj);

      if (thisType != otherType)
        return string.Compare(thisType.ToString(), otherType.ToString(), StringComparison.Ordinal);

      var other = (ValueObject)obj;

      object[] components = GetEqualityComponents().ToArray();
      object[] otherComponents = other.GetEqualityComponents().ToArray();

      for (int i = 0; i < components.Length; i++)
      {
        int comparison = CompareComponents(components[i], otherComponents[i]);
        if (comparison != 0)
          return comparison;
      }

      return 0;
    }

    private int CompareComponents(object object1, object object2)
    {
      if (object1 is null && object2 is null)
        return 0;

      if (object1 is null)
        return -1;

      if (object2 is null)
        return 1;

      if (object1 is IComparable comparable1 && object2 is IComparable comparable2)
        return comparable1.CompareTo(comparable2);

      return object1.Equals(object2) ? 0 : -1;
    }

    public int CompareTo(ValueObject other)
    {
      return CompareTo(other as object);
    }

    public static bool operator ==(ValueObject a, ValueObject b)
    {
      if (a is null && b is null)
        return true;

      if (a is null || b is null)
        return false;

      return a.Equals(b);
    }

    public static bool operator !=(ValueObject a, ValueObject b)
    {
      return !(a == b);
    }

    internal static Type GetUnproxiedType(object obj)
    {
      const string EFCoreProxyPrefix = "Castle.Proxies.";
      const string NHibernateProxyPostfix = "Proxy";

      Type type = obj.GetType();
      string typeString = type.ToString();

      if (typeString.Contains(EFCoreProxyPrefix) || typeString.EndsWith(NHibernateProxyPostfix))
        return type.BaseType;

      return type;
    }
  }
}