Builder: Valid Orders Only

OrderReq has required fields and derived invariants that are too easy to get wrong by hand. Build a **Builder** whose Build() enforces them or throws: - WithCustomer(string) — required, non-empty; Build() throws if missing - WithItem(string sku, int qty, decimal price) — repeatable; qty must be ≥ 1 (throw ArgumentException in WithItem itself) - WithRush() — flags rush delivery - Build() throws when: no customer, no items, or a rush order with more than 10 total items (business rule) Expose the built fields the tests read: Customer, TotalQty, Items (count), Rush. ``csharp public static class Solution { public sealed class OrderReq { public string Customer = ""; public int TotalQty; public int Items; public bool Rush; } public sealed class OrderBuilder { public OrderBuilder WithCustomer(string c); public OrderBuilder WithItem(string sku, int qty, decimal price); // qty >= 1 public OrderBuilder WithRush(); public OrderReq Build(); // enforces invariants } } ``