private void AddUserButton_Click(object sender, EventArgs e) { UsersListForm users_list = new UsersListForm(client.GetUsersList().Data.Split(' ') .Where(u => !UserList.Items.Contains(u)).ToArray()); users_list.ShowDialog(); if (users_list.SelectedUser != null) { ActionResult res = client.AddUserToContacts(users_list.SelectedUser); if (res.Result) { UserList.Items.Add(users_list.SelectedUser); } else { MessageBox.Show(ErrorMessages.ERR_UNDEFINED_RESPONSE, ErrorMessages.ERR_ERROR, MessageBoxButtons.OK, MessageBoxIcon.Error); } } }