I had this fragment of code:

NHibernate.Cfg.Configuration conf = new 
NHibernate.Cfg.Configuration();

conf.AddAssembly("NHibernateTesting");

And I got this exception at the second line:
"The dialect was not set. Set the property hibernate.dialect."

The problem was that my app.config was renamed to app.config.xml, 
and that is incorrect, the file must be just "app.config". 

I hope that it has been useful.

Nahuel from Argentina.