How do you preview a proration using Stripe.NET?

Question

I'm following the example provided in the Stripe documentation on Previewing Proration using the Stripe.NET library to try to find the amount that will be charged when a customer upgrades from Plan A to Plan B.

However, when I use the code sample in the documentation, I get an error:

UpcomingInvoiceOptions options = new UpcomingInvoiceOptions()
{
    CustomerId = "cus_XXXXXXXXXXXXX",
    SubscriptionProrationDate = DateTime.UtcNow,
    SubscriptionItems = new List<InvoiceSubscriptionItemOptions>()
    {
        new InvoiceSubscriptionItemOptions()
        {
            Id = "si_XXXXXXXXXXXXXX", // Current Sub Item
            PlanId = "plan_XXXXXXXXXXXX" // New plan
        }
    }
};
InvoiceService service = new InvoiceService();
var result = service.Upcoming(options);

The last line throws a Stripe.StripeException: You cannot update a subscription item without a subscription.

Answer 1

Turns out options.SubscriptionId is a required field for this action if you don't call service.Get first.

The following code produces the correct results:

UpcomingInvoiceOptions options = new UpcomingInvoiceOptions()
{
    CustomerId = "cus_XXXXXXXXXXXXX",
    SubscriptionId = "sub_XXXXXXXXXXXX",
    SubscriptionProrationDate = DateTime.UtcNow,
    SubscriptionItems = new List<InvoiceSubscriptionItemOptions>()
    {
        new InvoiceSubscriptionItemOptions()
        {
            Id = "si_XXXXXXXXXXXXXX", // Current Sub Item
            PlanId = "plan_XXXXXXXXXXXX" // New plan
        }
    }
};
InvoiceService service = new InvoiceService();
var result = service.Upcoming(options);