Refactored the using-Statement to a seperate method. Reduced code smell in DAO-methods.

RefactoredUsingOfTransaction.cs

public class EntityOperation : IEntityOperation
{
  readonly ISession _session;

  public EntityOperation(ISession session)
  {
    _session = session;
  }

  public void Save(object entity)
  {
    Commit(
      () => _session.Save(entity));
  }

  public void Update(object entity)
  {
    Commit(
      () => _session.Update(entity));
  }

  public void Delete(object entity)
  {
    Commit(
      () => _session.Delete(entity));
  }

  private void Commit(Action action)
  {
    using (var t = _session.BeginTransaction())
    {
      action.Invoke();
      t.Commit();
    }
  }
}