Handlers.cs

public class PayOrderHandler
{
    private readonly DbContext _db;

    public PayOrderHandler(DbContext db)
    {
        _db = db;
    }
    
    public async Task Handle(PayOrder cmd)
    {
        using var trx = _db.BeginTransaction();

        _db.Inbox.Add(new InboxMessage(cmd.MessageId));

        var order = _db.Orders.Find(cmd.OrderId);
        order.MarkPaymentPending(cmd.PaymentAttemptId);

        _db.Outbox.Add(new OutboxMessage(
            type: "ChargePayment",
            key: cmd.PaymentAttemptId,
            payload: new { cmd.OrderId, cmd.Amount }
        ));

        await _db.SaveChangesAsync();
        await trx.CommitAsync();
    }
}

public class ChargePaymentHandler
{
    private readonly PaymentGateway _paymentGateway;
    private readonly DbContext _db;

    public ChargePaymentHandler(PaymentGateway paymentGateway, DbContext db)
    {
        _paymentGateway = paymentGateway;
        _db = db;
    }
    
    public async Task Handle(ChargePayment cmd)
    {
        var receipt = await _paymentGateway.Charge(
            orderId: cmd.OrderId,
            amount: cmd.Amount,
            idempotencyKey: cmd.MessageId // if supported
        );

        using var trx = _db.BeginTransaction();

        var order = _db.Orders.Find(cmd.OrderId);
        order.MarkPaid(receipt.Id);
        
        await _db.SaveChangesAsync();
        await trx.CommitAsync();
    }
}