title: Null entities in DDD
body: <p>Take an example of a <strong>Product</strong> entity or aggregate which has a reference to another entity called <strong>Category</strong>. Before a domain event like <strong>CategorizeProduct</strong> happens, the Category's state is empty. To represent that in C#, I have to use <code>NULL</code>. I can't use <code>Nullable&lt;T&gt;</code> in C# because that can be used for value types only.</p>
  
  <p>Is using <code>NULL</code> in DDD an anti-pattern? Some of the solutions that have been suggested are to (1) use <code>MayBe&lt;T&gt;</code> (2) use a <a href="http://blog.ploeh.dk/2011/02/04/TheBCLalreadyhasaMaybemonad/" rel="nofollow">collection</a> in the place of single entity.</p>
  
  <p><code>MayBe&lt;t&gt;</code> is an option, but is there a different approach to handle Empty entity state in DDD?</p>