BranchingWithDiscriminatedUnions.cs

namespace DiscriminatedUnions
{
    using System.Collections.Generic;
    using CartState = Union3<CartStateEmpty, CartStateActive, CartStatePaid>;

    public class Union3<A, B, C>
    {
        public Union3(A item) { Item = item; }
        public Union3(B item) { Item = item; }
        public Union3(C item) { Item = item; }

        public dynamic Item { get; }
    }

    static class Program
    {
        static void Main(string[] args)
        {
            const decimal paidAmount = 12.34m;

            // no real need for discriminated unions if process is deterministic
            // just use the API to guard against unallowed transitions
            var paid = new CartStateEmpty()
                .AddProduct(Product.ProductX)
                .AddProduct(Product.ProductY)
                .Pay(paidAmount);

            // but discriminated unions quite useful if there's branching
            var empty = new CartState(new CartStateEmpty());
            var maybeAdded = empty.MaybeAddProduct(Product.ProductX);
            var maybeAdded2 = maybeAdded.MaybeAddProduct(Product.ProductY);
            var maybePaid = maybeAdded2.MaybePay(paidAmount);
        }

        public static CartState MaybeAddProduct(this CartState cartState, Product product) => cartState.Item switch
        {
            CartStateEmpty      => product.InStock ? new CartState( new CartStateActive(product) ) : cartState,
            CartStateActive csa => new CartState(csa.AddProduct(product)),
            _                   => cartState
        };

        public static CartState MaybePay(this CartState cartState, decimal amount) => cartState.Item switch
        {
            CartStateActive csa => amount <= 1000 ? new CartState(csa.Pay(amount)) : cartState,
            _                   => cartState
        };
    }

    public class Product
    {
        public static Product ProductX { get; } = new Product();
        public static Product ProductY { get; } = new Product();
        public bool InStock { get; }
    }

    class CartStateEmpty
    {
        public CartStateActive AddProduct(Product product) => new CartStateActive(product);
    }

    class CartStateActive
    {
        private IList<Product> _products;

        public CartStateActive(Product product)
        {
            _products = new List<Product>{ product };
        }

        public CartStateActive AddProduct(Product product)
        {
            _products.Add(product);
            return this;
        }

        public CartStatePaid Pay(decimal amount) => new CartStatePaid();
    }

    class CartStatePaid { }
}