C# Own SettingsProvider problem

Question

I am writing my own settings provider:

 public static class SettingsProvider
{
    private const string SettingsFilePath = "Settings.xml";
    private static ISettings _settings;

    public static ISettings Settings
    {
        get
        {
            if (_settings == null)
            {
               _settings = readSettings();
            }
            return _settings;
        }

        set
        {
            _settings = value;
            saveSettings(value);
        }
    }


    private static ISettings readSettings()
    {
        if (File.Exists(SettingsFilePath))
        {
            using (Stream stream = File.Open(SettingsFilePath, FileMode.Open))
            {
                var bf = new BinaryFormatter();
                return (ISettings)bf.Deserialize(stream);
            }
        }
        return new Settings();

    }

    private static void saveSettings(ISettings settings)
    {
        using (Stream stream = File.Open(SettingsFilePath, FileMode.Create))
        {
            var bf = new BinaryFormatter();
            bf.Serialize(stream, settings);
        }
    }
}

I want use it like that:

var settings = SettingsProvider.Settings; //get settings from xml 
var settings2 = SettingsProvider.Settings; //second time, i dont need to look to xml again

SettingsProvider.Settings = new Settings(); //Settings() implemented ISettings

I have a problem with assignment new settings, method of property Settings is never called, so is not saved to the xml. How to do that?