Why should you care?

ShopNest orders (Pending → Paid → Shipped → Cancelled) have different allowed actions per state.

See it live — copy this example

Paste into a C# console or class library project and run dotnet run.

public interface IOrderState { void Pay(Order o); void Ship(Order o); }
public sealed class Order
{
    public IOrderState State { get; set; } = new PendingState();
    public void Pay() => State.Pay(this);
    public void Ship() => State.Ship(this);
}
public sealed class PendingState : IOrderState
{
    public void Pay(Order o) { Console.WriteLine("paid"); o.State = new PaidState(); }
    public void Ship(Order o) => Console.WriteLine("cannot ship yet");
}
public sealed class PaidState : IOrderState
{
    public void Pay(Order o) => Console.WriteLine("already paid");
    public void Ship(Order o) { Console.WriteLine("shipped"); o.State = new ShippedState(); }
}
public sealed class ShippedState : IOrderState
{
    public void Pay(Order o) => Console.WriteLine("noop");
    public void Ship(Order o) => Console.WriteLine("already shipped");
}

var order = new Order();
order.Pay();
order.Ship();

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• PendingState allows Pay; PaidState allows Ship.
• Transitions assign a new state object.
• Illegal calls become no-ops or errors inside the state.

Practice next

1. Pay then Ship successfully.
2. Try Ship first and see refusal.
3. Add CancelledState.
4. Throw on illegal Ship from Pending.
5. Log every transition.

Remember

Behavior per state object. Clear transitions. Replace mega-switches.

ShopNest order lifecycle

Domain Order delegates to state objects.

Outcome: Illegal transitions blocked in one place.