[摘抄]NHibernate: Nullable DateTime Issues

Url:http://ayende.com/Blog/archive/2007/03/26/NHibernate-Nullable-DateTime-Issues.aspx

System.DateTime is a value type in .Net, which means that it can never be null. But what happens when you have a nullable date time in the database, and you load it into a DateTime type?

Consider this simple example. Mapping:

<property name="UpdatedDate" nullable="True"  column="updated_date" type="DateTime" />

Property:

public DateTime UpdatedDate 
{
      get { return m_dateTime; }
      set { m_DateTime = value; }
}

When NHibernate loads a null value from the database, it cannot put a null in the UpdatedDate property, the CLR doesn't allow it. What happens is that the UpdatedDate property is set to the default DateTime value, in this case: 01/01/0001.

This can cause two major issues down the road. The first is that 01/01/0001 is not a valid date in SQL Server, so when you try to save the value, it will throw an exception. The second is that because of this issue, when NHibernate needs to track changes, it will check if null != 01/01/0001, and it will turn out that yes, this entity (which we never touched) has changed.

This can cause an extra update (and thus an exception) which can cause some fairly significant head scratching. The solution is simple, you need to let NHibernate know what to do with null values. This can be done by simply using a nullable type, such as:

public DateTime? UpdatedDate 
{
      get { return m_dateTime; }
      set { m_DateTime = value; }
}

Or by using the Nullables.dll library for 1.1, and specifying the correct type in the mapping:

<property name="UpdatedDate" column="updated_date" type="Nullables.NHibernate.NullableDateTimeType, Nullables.NHibernate" />

With this property:

public Nullables.NullableDateTime UpdatedDate 
{
   get { return m_dateTime; }
   set { m_DateTime = value; }
}

Thanks for Sheraz Khan, for finding out and brining this to my attention, since then, I have run into the issue a couple of times, and I hope that if I blog about it, I will remember to match nullabilities.