I asked this question yesterday: https://softwareengineering.stackexchange.com/questions/364895/entity-and-value-object-are-not-part-of-the-ubiqtious-language-should-this-stop. I originally decided to introduce a ValueObject class as described here: https://lostechies.com/jimmybogard/2007/06/25/generic-value-object-equality/, which contained equality methods for all value objects i.e. all Value Objects inherited from ValueObject the value object class. However, this approach was criticised.

Please see the code below:

public sealed class Surname : IEquatable<Surname>     {         private readonly string _value;         public Surname(string surname)         {             if (surname.Length==0)                 throw new ArgumentException("Invalid value.", "Surname");             this._value = surname;         }          public string Value         {             get { return _value; }         }          public static bool operator ==(Surname surname1, Surname surname2)         {             if (!ReferenceEquals(surname1, null) &&                 ReferenceEquals(surname2, null))             {                 return false;             }             if (ReferenceEquals(surname1, null) &&                  !ReferenceEquals(surname2, null))             {                 return false;             }             return surname1.Equals(surname2);         }          public static bool operator !=(Surname surname1, Surname surname2)         {             return !(surname1== surname2);         }          public bool Equals(Surname other)         {             if (other != null)             {                 return _value == other._value;             }             return base.Equals(other);         }          public override bool Equals(object obj)         {             return Equals(obj as Surname);         }          public override int GetHashCode()         {             unchecked             {                 int hash = 17;                 hash = hash * 23 + _value.GetHashCode();                 return hash;             }         }     } 

It is a DDD Value Object for a Surname.

I have added some methods for equality and:

Q1) I would like some feedback on the quality of them.

Q2) This question (point one) links to a presentation by Eric Evans where he appears to advise against introducing equality for Entities. Is it advisable to always compare entities by reference these days?