Commands und Handler mit MediatR

builder.Services.AddMediatR(cfg =>
    cfg.RegisterServicesFromAssembly(typeof(Program).Assembly));

public record CreateOrderCommand(int CustomerId, string[] Items)
    : IRequest<int>;   // returns the new order id

public record CancelOrderCommand(int OrderId) : IRequest;

public class CreateOrderHandler
    : IRequestHandler<CreateOrderCommand, int>
{
    public Task<int> Handle(
        CreateOrderCommand request, CancellationToken ct)
    {
        // create the order...
        return Task.FromResult(newOrderId);
    }
}

public class CreateOrderHandler(AppDbContext db)
    : IRequestHandler<CreateOrderCommand, int>
{
    public async Task<int> Handle(
        CreateOrderCommand request, CancellationToken ct)
    {
        var order = new Order { CustomerId = request.CustomerId };
        db.Orders.Add(order);
        await db.SaveChangesAsync(ct);
        return order.Id;
    }
}

public class OrdersController(ISender sender) : ControllerBase
{
    [HttpPost]
    public async Task<IActionResult> Create(CreateOrderCommand cmd)
    {
        int id = await sender.Send(cmd);
        return CreatedAtAction(nameof(Get), new { id }, null);
    }
}

app.MapPost("/orders", async (
    CreateOrderCommand cmd, ISender sender) =>
{
    var id = await sender.Send(cmd);
    return Results.Created($"/orders/{id}", new { id });
});

public record GetOrderQuery(int Id) : IRequest<OrderDto?>;

public class GetOrderHandler(AppDbContext db)
    : IRequestHandler<GetOrderQuery, OrderDto?>
{
    public async Task<OrderDto?> Handle(
        GetOrderQuery q, CancellationToken ct) =>
        await db.Orders
            .Where(o => o.Id == q.Id)
            .Select(o => new OrderDto(o.Id, o.Total))
            .SingleOrDefaultAsync(ct);
}

// CreateOrderCommand -> exactly one CreateOrderHandler

public record Result(bool Ok, string? Error);
public record PlaceOrderCommand(int Id) : IRequest<Result>;

var handler = new CreateOrderHandler(inMemoryDb);
var id = await handler.Handle(
    new CreateOrderCommand(1, ["sku-1"]), CancellationToken.None);
Assert.True(id > 0);