Newer C# version for https://fsharpforfunandprofit.com/posts/designing-for-correctness/

BasicCart.cs

using System;
using System.Collections.Immutable;

namespace ConsoleDU
{
    record CartItem(string Value);

    record Payment(decimal Amount);

    abstract record Cart
    {
        public record Empty () : Cart
        {
            public new static Active Add(CartItem item) => new(ImmutableList.Create(item));
        }
        public record Active (ImmutableList<CartItem> UnpaidItems) : Cart
        {
            public new Active Add(CartItem item) => this with {UnpaidItems = UnpaidItems.Add(item)};
            public new Cart Remove(CartItem item) => this with {UnpaidItems = UnpaidItems.Remove(item)} switch
            {
                var (items) when items.IsEmpty => new Empty(),
                { } active => active
            };

            public new Cart Pay(decimal amount) => new PaidFor(UnpaidItems, new(amount));
        }
        public record PaidFor (ImmutableList<CartItem> PaidItems, Payment Payment) : Cart;

        public Cart Display()
        {
            Console.WriteLine(this switch
            {
                Empty => "Cart is Empty",
                Active cart => $"Cart has {cart.UnpaidItems.Count} items",
                PaidFor(var items, var payment) => $"Cart has {items.Count} paid items. Amount paid: {payment.Amount}",
                _ => "Unknown"
            });
            return this;
        }

        public Cart Add(CartItem item) => this switch
        {
            Empty state => Empty.Add(item),
            Active state => state.Add(item),
            _ => this
        };

        public static Cart NewCart => new Empty();

        public Cart Remove(CartItem item) => this switch
        {
            Active state => state.Remove(item),
            _ => this
        };

        public Cart Pay(decimal amount) => this switch
        {
            Active cart => cart.Pay(amount),
            _ => this
        };
    }

    class Program
    {
        static void Main(string[] args)
        {
            Cart.NewCart
                .Display()
                .Add(new("apple"))
                .Add(new("orange"))
                .Display()
                .Remove(new("orange"))
                .Display()
                .Remove(new("apple"))
                .Display()
                .Add(new("orange"))
                .Pay(23M)
                .Display();
            ;
        }
    }
}