Thursday, April 12, 2012

NHibernate: DateTime support

By default, if you load a datetime value from the database through NHibernate, the DateTimeKind will be set to ‘Unspecified’. For a specific requirement on a recent project, the DateTimeKind needed to be set to ‘Local’ instead.
So I dived into NHibernate to discover a rich set of Date/Time related functionality. Let’s create a simple object:
public class SampleEntity 
 public DateTimeEntity() 
  CreatedOn = DateTime.Now;    

 public virtual Guid Id { get; private set; }    
 public virtual DateTime CreatedOn { get; set; }

If we save this entity to the database and load it again, we see there’s a difference:

Original entity:
Id: 9245fe4a-d402-451c-b9ed-9c1a04247482
CreatedOn: 2012-04-08 11:57:22 PM (Local)

Reloaded entity:
Id: 9245fe4a-d402-451c-b9ed-9c1a04247482
CreatedOn: 2012-04-08 11:57:22 PM (Unspecified)

When creating the entity, we initialized the CreatedOn property with DateTime.Now.(This is always the local time.) After saving and reloading the object to the database, the DateTimeKind is changed to ‘Unspecified’. This is because the database does not store whether the DateTime value is Local or UTC. NHibernate has no way of knowing which one it is, hence Unspecified.

To solve this NHibernate 3 includes two new DbTypes. In our mapping file, we can specify one of the following types:

<property name="CreatedOn" type="LocalDateTime"/>
<property name="CreatedOn" type="UtcDateTime"/>

By doing this we are explicitly telling NHibernate whether the database stores Local or UTC times.
For more information I recommend this blogpost by James Kovacs.

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