public virtual T Update(T item, int id)
    {
        if (item == null)
            throw new ArgumentNullException(string.Format("El {0} no puede ser nulo!", typeof(T).Name.ToString()));

        SessionManager.BeginTransaction();
        try
        {
            SessionManager.CurrentSession.Update(item);
            SessionManager.CommitTransaction();
            return item;
        }
        catch (Exception ex)
        {
            SessionManager.RollbackTransaction();
            throw ex;
        }
    }

public static void BeginTransaction()
    {
        CurrentSession.BeginTransaction();
    }

    public static void CommitTransaction()
    {
        if (CurrentSession.Transaction != null && CurrentSession.Transaction.IsActive)
            CurrentSession.Transaction.Commit();
        CurrentSessionContext.Unbind(Factory);
        CurrentSession.Close();
    }

    public static void RollbackTransaction()
    {
        if (CurrentSession.Transaction != null && CurrentSession.Transaction.IsActive)
            CurrentSession.Transaction.Rollback();
        CurrentSessionContext.Unbind(Factory);
        CurrentSession.Close();
    }