title: nhibernate: what are the best practices for implementing equality?
body: <p>I think that Entities should implement equality by primary key comparison as default, but the nhibernate documentation recommends using business identity:</p>
  
  <blockquote>
    <p>The most obvious way is to implement Equals()/GetHashCode()  by comparing the identifier value of both objects. If the value is the same, both must be the same database row, they are therefore equal (if both are added to an ISet, we will only have one element in the ISet). Unfortunately, we can't use that approach. NHibernate will only assign identifier values to objects that are persistent, a newly created instance will not have any identifier value! We recommend implementing Equals() and GetHashCode() using Business key equality.</p>
    
    <p>Business key equality means that the Equals() method compares only the properties that form the business key, a key that would identify our instance in the real world (a natural candidate key)</p>
  </blockquote>
  
  <p>And the example (also from the doc):</p>
  
  <pre><code>public override bool Equals(object other)
  {
      if (this == other) return true;
  
      Cat cat = other as Cat;
      if (cat == null) return false; // null or not a cat
  
      if (Name != cat.Name) return false;
      if (!Birthday.Equals(cat.Birthday)) return false;
  
      return true;
  }
  </code></pre>
  
  <p>This got my head spinning because the notion of business identity (according to the example) is the same as comparison by syntax, which is basically the type of semantics I associate with ValueObjects. The reason for not using database primary keys as comparison values is because this will change the hashcode of the object if the primary key is not generated on the client side (for ex incremental) and you use some sort of hashtable collection (such as ISet) for storing your entities.</p>
  
  <p><b>How can I create a good equality implementation which does not break the general rules for equality/hashcode (<a href="http://msdn.microsoft.com/en-us/library/bsc2ak47.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/bsc2ak47.aspx</a>) and conforms to nhibernate rules as well?</b></p>