SliderThumbToolTipConverterSample.cs

    {
        // somewhere
        Slider.ThumbToolTipValueConverter = new SliderValueConverter() { Labels = new List<string>(){"a", "b", "c"} };
    }
    
    public class SliderValueConverter : IValueConverter
    {
        public List<string> Labels { get; set; }

        public object Convert(object value, Type targetType, object parameter, string language)
        {
            var selected = (int)Math.Round((double)value);
            if (Labels == null || selected >= Labels.Count) { return value.ToString(); }

            return Labels[selected];
        }

        public object ConvertBack(object value, Type targetType, object parameter, string language)
        {
            throw new NotImplementedException();
        }
    }