How to set a configuration property when using fluent nhibernate?

You can use the method ExposeConfiguration on a FluentConfiguration instance, to access the original NHibernate Configuration object.

Then, you'll have access to the Properties property, and you will be able to add the current_session_context_class one.

Here is a the pseudo-code:

Fluently.Configure()
   .Database(SQLiteConfiguration.Standard.InMemory)
   .ExposeConfiguration(c =>
                        {
                          c.Properties.Add("current_session_context_class", 
                                           typeof(YourType).FullName);
                        })
   //.AddMapping, etc.
   .BuildSessionFactory();