How to Implement a Singleton class

Question Comments

I have found a bug for this singleton implementation:
If you have no records in db follow code create several records:
Singleton.GetInstance(session); // For example for default creation
Singleton.GetInstance(session); // For exapmle for set some settings

So I have fixed it with this code:
public static Singleton GetInstance(Session session)
{
var result = session.QueryInTransaction<Singleton>().FirstOrDefault();
if (result == null)
{
result = new Singleton(session)
{
// some default init
}
}

return result;
}

The key is using QueryInTransaction, but as far I understand it doesn't prevent using several sessions here.