Applying SOLID and design patterns in LLD — Low-Level Design (LLD) — Object Design, UML and Classic Problems in C#

3. Applying SOLID and design patterns in LLD

Intermediate★ Must knowLesson 3 of 8

Situation in a design	Pattern	Example
Behaviour varies by type/rule and can be swapped	Strategy	Fee calculation per vehicle type; payment methods; pricing
Creating objects depends on input	Factory / Abstract Factory	`VehicleFactory.Create("CAR", number)`
Object behaves differently in each lifecycle state	State	Vending machine (Idle → HasMoney → Dispensing), order status
Notify many parts when something happens	Observer	Display boards update when spots change; notify users
Exactly one shared instance	Singleton (prefer DI singleton lifetime)	`ParkingLot` instance, configuration
Add features without changing a class	Decorator	Add-ons to a pizza/coffee price; logging around a service
Step-by-step construction of complex objects	Builder	Building a `Booking` or HTTP request
Chain of handlers each deciding to handle or pass on	Chain of Responsibility	ATM cash dispenser (2000 → 500 → 100 notes), approval workflows
Encapsulate a request as an object	Command	Undo/redo, queued elevator requests
Simplify a complex subsystem	Facade	`BookingFacade` hides seats, payment, notifications

csharp

// Strategy + Open/Closed: add a new vehicle fee rule without touching existing code
public interface IFeeStrategy { decimal Calculate(TimeSpan duration); }
public sealed class CarFee : IFeeStrategy { public decimal Calculate(TimeSpan d) => 40 + Math.Max(0, Math.Ceiling(d.TotalHours) - 1) * 30; }
public sealed class BikeFee : IFeeStrategy { public decimal Calculate(TimeSpan d) => 20 + Math.Max(0, Math.Ceiling(d.TotalHours) - 1) * 10; }

public sealed class FeeCalculator(IReadOnlyDictionary<VehicleType, IFeeStrategy> strategies)
{
    public decimal For(VehicleType type, TimeSpan duration) => strategies[type].Calculate(duration);
}