private void Reject_Click(object sender, RoutedEventArgs e)
        {
            Button         senderButton = sender as Button;
            IdentityResult result;
            AppUser        user;

            if (senderButton.DataContext is AppUser)
            {
                user   = (AppUser)senderButton.DataContext;
                result = helper.RejectRegisterRequest(user);
                if (result.AreSameWith(IdentityResult.Success()))
                {
                    MessageBox.Show("拒绝用户审批成功!");
                    approvallist.Remove(user);
                    approval_list.ItemsSource = null;
                    approval_list.ItemsSource = approvallist;
                }
                else
                {
                    MessageBox.Show(string.Join(",", result.Errors));
                }
            }
            else
            {
                result = IdentityResult.Error("待审批的用户不存在.");
                MessageBox.Show("错误:待审批的用户不存在.");
            }
        }