A.Supple_Design_Declarative_Programming.cs

    public class Customer // PRE REFACTORING
    {
        public decimal EarnedRewards { get; private set; }

        public int TotalPoints { get; private set; }

        public DateTime MemberSince { get; private set; }

        public void CalculateRewards()
        {
            var pointLimit = 5000;

            if (EarnedRewards > 50)
                pointLimit = 10000;

            if (TotalPoints > pointLimit)
            {
                var rewardValue = 5m;

                if (DateTime.UtcNow.Year - MemberSince.Year >= 5)
                    rewardValue *= 2;

		TotalPoints = TotalPoints - pointLimit; //Side Effect!
                EarnedRewards = rewardValue; //Side Effect!
            }
        }
    }

B.Refactored_Rewards.cs

public class Rewards //Refactoring to an immutable Value Object
{
	public decimal EarnedRewards { get; }

	public int Points { get; }

	public Rewards(decimal earnedRewards, int points)
	{
		EarnedRewards = earnedRewards;
		Points = points;
	}

	public Rewards Earn(Rewards other)
	{
		if (other == default(Rewards))
			return this;

		return new Rewards(EarnedRewards + other.EarnedRewards,
			                   other.Points);
	}

	public Rewards RedeemPoints(int memberShipYears)
	{
		var pointLimit = 5000;

		if (EarnedRewards > 50)
			pointLimit = 10000;

		if (Points < pointLimit)
			return default(Rewards);

		var rewardValue = 5m;

		if (memberShipYears >= 5)
			rewardValue *= 2;

		return new Rewards(rewardValue, Points - pointLimit);
	}
}    

C.Refactored_Customer.cs

    public class Customer
    {
        public Rewards Rewards { get; private set; }

        public DateTime MemberSince { get; }

        public int MemberShipYears => DateTime.UtcNow.Year - MemberSince.Year;

        public void AdjustRewards()
        {
            Rewards = Rewards.Earn(Rewards.RedeemPoints(MemberShipYears)); //Declarative Style
        }
    }