How to modify NHibernate mappings at Runtime

When you have the need to modify or change your NHibernate mappings at runtime the obvious choice would be to go for one of a new generation mapping engine (like ConfORM, Fluent NHibernate, or anything else), but what about you plain good old XML mapping files you still use?

Changing them was a nightmare before NHibernate 3...I gave up on the task when I thought I had to pre-load the XML files, parse them, modify them in memory and the feed them to NHibernate; these operations seemed too long and too ‘hacky’ for a task that should have been simple.

NHibernate 3 comes to the rescue and offers a nice couple of events you can use to inspect and change your mapping before and after they are accepted and bound to be used by the configuration object:

BeforeBindMapping(object sender, BindMappingEventArgs) - this event is raised before the mapping is actually tied to the configuration object, you can inspect the mapping properties and change their values. Once the mapping is bound it becomes immutable.
AfterBindMapping(object sender, BindMappingEventArgs) - this event is raised after the mapping is bound to the configuration object, you can use it to inspect the metadata and get some information.
Using these events you have a chance to alter the mapping (acting directly with the NHibernate metadata structures) at runtime, whatever is the mapping method you chosen; in fact these events will be fired when you add the mappings to the NHibernate Configuration object via configuration.AddAssembly(), configuration.AddDeserializedMapping() and so on...

All the information are encapsulated in the BindMappingEventArgs class, which is something like this:

public class BindMappingEventArgs : EventArgs
{

public NHibernate.Dialect.Dialect Dialect { get; private set; }
public string FileName { get; private set; }
public HbmMapping Mapping { get; private set; }

}

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