Distance.cs

using System;
using Xunit;

namespace ValueObject
{
    public abstract record UnitOfMeasure;

    public record Kilometers : UnitOfMeasure;

    public record Miles : UnitOfMeasure;

    public record Distance
    {
        public UnitOfMeasure UnitOfMeasure { get; }
        public decimal Value { get; }

        public Distance(UnitOfMeasure unitOfMeasure, decimal value)
        {
            UnitOfMeasure = unitOfMeasure;
            Value = value;
        }

        public Distance ToMiles()
        {
            switch (UnitOfMeasure)
            {
                case Miles:
                    return new Distance(new Miles(), Value);
                case Kilometers:
                    return new Distance(new Miles(), Math.Round(Value * 0.621371m, 2, MidpointRounding.AwayFromZero));
                default:
                    throw new InvalidOperationException();
            }
        }

        public Distance ToKilometers()
        {
            switch (UnitOfMeasure)
            {
                case Miles:
                    return new Distance(new Kilometers(), Math.Round(Value * 1.60934m, 2, MidpointRounding.AwayFromZero));
                case Kilometers:
                    return new Distance(new Kilometers(), Value);
                default:
                    throw new InvalidOperationException();
            }
        }
    }

    public class DistanceTest
    {
        [Fact]
        public void Test()
        {
            var distance1 = new Distance(new Kilometers(), 100);
            var distance2 = distance1.ToMiles();
            var distance3 = distance2.ToKilometers();

            Assert.Equal(distance3, distance1);
        }
    }
}