An example MVC controller for interacting with a PayPal payment service

gistfile1.cs

namespace TheBush.Controllers
{
    using System.Globalization;
    using System.IO;
    using System.Web.Mvc;

    using PayPal.Exception;

    using Application.Services;
    
    using System.Linq;
    using global::PayPal.Api.Payments;

    public class ShoppingCartController : Controller
    {
        private readonly IPaymentService _paymentService;

        public ActionResult Purchase()
        {
            var baseUrl = string.Concat(this.Request.Url.Scheme, "://", this.Request.Url.Authority);

            try
            {
                var order = // Retrieve/create your order object somehow

                var cancelUrl = this.Url.Action("purchaseunsucessful", "order");
                var returnUrl = this.Url.Action("purchasecomplete", "order", new { orderNumber = order.OrderNumber.ToLower() });

                var payment = this._paymentService.CreatePayment(order, string.Concat(baseUrl, cancelUrl), string.Concat(baseUrl, returnUrl));

                if (!payment.WasCreated())
                {
                    return this.RedirectToAction("purchaseunsucessful");
                }
                
                // Save order with payment.id somewhere on it

                return this.Redirect(payment.GetApprovalUrl());
            }
            catch (PayPalException ex)
            {
                this.ViewBag.Error = ((ConnectionException)ex.InnerException).Response;
            }
            catch (Exception ex)
            {
                this.ViewBag.Error = ex.Message;
            }

            return this.RedirectToAction("purchaseunsucessful");
        }

        public ActionResult PurchaseUnsucessful()
        {
            return this.View();
        }

        public ActionResult PurchaseComplete(string orderNumber, string payerId)
        {
            try
            {
                var order = // Retrieve the order using the order number we saved into the returnUrl

                this._paymentService.ConfirmPayment(order.PaymentReference, payerId, order);

                return this.View("purchasecomplete", order);
            }
            catch (PayPalException ex)
            {
                this.ViewBag.Error = ((ConnectionException)ex.InnerException).Response;
            }
            catch (Exception ex)
            {
                this.ViewBag.Error = ex.Message;
            }

            return this.RedirectToAction("purchaseunsucessful");
        }
    }
    
    public static class PaymentExtensions
    {
        public static string GetApprovalUrl(this Payment payment)
        {
            return payment.links.First(link => link.rel.ToLowerInvariant() == "approval_url".ToLowerInvariant()).href;
        }

        public static bool WasSucessful(this Payment payment)
        {
            return payment.state.ToLowerInvariant() == "approved";
        }

        public static bool WasCreated(this Payment payment)
        {
            return payment.state.ToLowerInvariant() == "created";
        }
    }
}