private void UserValueValidating(object sender, CancelEventArgs e) { TextBox userValue = sender as TextBox; if (userValue == null) { return; } IConfigurationItem configurationItem = userValue.Tag as IConfigurationItem; if (configurationItem != null) { if (!configurationItem.IsValidUserInput(userValue.Text)) { userValue.BackColor = Color.Red; e.Cancel = true; } else { userValue.BackColor = new TextBox().BackColor; } } }