NHibernate Pitfalls: Specifying Property Types

This is part of a series of posts about NHibernate Pitfalls. See the entire collection here.

When you want to specify an NHibernate type (implementation of NHibernate.Type.IType) for a property using mapping by code, you might be tempted to do something like this:

   1: ca.Property(x => x.SomeProperty, x =>

   2: {

   3:     x.Type<StringClobType>();

   4: });

If you try this, you will get a nasty exception about the type not having a public parameterless constructor. Ouch!

The thing is: NHibernate types are supposed to be stateless, and therefore it does not make sense to have several instances of a type, that is why NHibernate encourages us to use the static fields defined in the NHibernateUtil class and that is also why most of the built-in types don’t have a public parameterless constructor.

So, if you want to implement your own types, you can certainly add a public parameterless constructor to them and use the above syntax, but if you are going to use the built-in types, you should instead use:

   1: ca.Property(x => x.SomeProperty, x =>

   2: {

   3:     x.Type(NHibernateUtil.StringClob);

   4: });