Messages & slices

// An intention sent to exactly one owner
public sealed record ChargePayment(string OrderId, decimal Amount) : ICommand;

// An internal fact inside one bounded context
public sealed record PaymentCharged(string OrderId, decimal Amount) : IDomainEvent;

// A versioned public contract across boundaries
[IntegrationEvent("payments.payment-charged", Version = 1)]
public sealed record PaymentChargedV1(string OrderId, decimal Amount, string Currency)
    : IIntegrationEvent;

[Union]
public abstract record Payment : IState<Payment>
{
    public static Payment Initial { get; } = new None();

    public Payment Evolve(IDomainEvent domainEvent) => (this, domainEvent) switch
    {
        (None, PaymentCharged charged) => new Charged(charged.Amount),
        (Charged, PaymentRefunded)     => new Refunded(),
        _                              => this,
    };

    public sealed record None : Payment;
    public sealed record Charged(decimal Amount) : Payment;
    public sealed record Refunded : Payment;
}

public sealed class ChargePaymentSlice : Slice<ChargePayment, Payment, PaymentCharged, PaymentError>
{
    public override object KeyOf(ChargePayment command) => command.OrderId;

    public override Result<PaymentCharged, PaymentError> Decide(ChargePayment command, Payment payment) => payment switch
    {
        Payment.None when command.Amount > 1_000m => PaymentError.CardDeclined,
        Payment.None                              => new PaymentCharged(command.OrderId, command.Amount),
        Payment.Charged or Payment.Refunded       => PaymentError.AlreadyCharged,
    };
}