SettingsSavedEventArgs.FailedSettingsList Property

private void AttachSettingsSavedEventHandler()
{
    ((ClientSettingsProvider)Properties.Settings.Default.Providers
        ["System.Web.ClientServices.Providers.ClientSettingsProvider"])
        .SettingsSaved +=
        new EventHandler<SettingsSavedEventArgs>(Form1_SettingsSaved);
}

private void Form1_SettingsSaved(object sender,
    SettingsSavedEventArgs e)
{
    // If any settings were not saved, display a list of them. 
    if (e.FailedSettingsList.Count > 0)
    {
        String failedSettings = String.Join(
            Environment.NewLine,
            e.FailedSettingsList.ToArray());

        String message = String.Format("{0}{1}{1}{2}",
            "The following setting(s) were not saved:",
            Environment.NewLine, failedSettings);

        MessageBox.Show(message, "Unable to save settings",
            MessageBoxButtons.OK, MessageBoxIcon.Warning);
    }
}