public class UnitConvert
    {
        private enum UnitsValue
        {
            mm = 1,
            cm = 10,
            dm = 100,
            m = 1000,
            km = 1000000
        }

        private Dictionary<string, int> UnitsDictionary = new Dictionary<string, int>();

        public UnitConvert()
        {
            foreach (var item in Enum.GetValues(typeof(UnitsValue)))
            {
                UnitsDictionary.Add(item.ToString(), (int)item);
            }
        }

        public UnitAndDistance ConvertAnUnitDistance(UnitAndDistance entryDistance, string toUnit)
        {
            if (UnitsDictionary.Any(x => x.Key == toUnit))
            {
                double convertedValue = (entryDistance.Value * UnitsDictionary
                    .FirstOrDefault(x => x.Key == entryDistance.Unit)
                    .Value) / UnitsDictionary.FirstOrDefault(x => x.Key == toUnit).Value;
                return new UnitAndDistance(toUnit, convertedValue);
            }
            else
            {
                throw new IndexOutOfRangeException("Cannot find that unit!");
            }
        }
    }