Quick demo - bare minimum to process a payment through paypal

PayPalSaleDemo.cs

using PayPal.Api;
using System;
using System.Collections.Generic;

namespace PaypalPayment2
{
    /*
     Instructions:
     1. Install-Package PayPal
     2. Set your clientId and clientSecret parameters in GetApiContext()
     3. Replace parameters like paymentId payerId returnUrl etc.
    */

    internal class Program
    {
        private static void Main(string[] args)
        {
            //// 1.
            ////CreatePaymentDemo();
            //// 2.
            ////ExecutePaymentDemo();
            //// Optional
            //ListPaymentsDemo();
        }

        private static APIContext GetApiContext()
        {
            // Authenticate with PayPal

            var config = new Dictionary<string, string>() {
                {"mode","sandbox" },
                {"clientId","" },
                {"clientSecret","" }
            };

            var accessToken = new OAuthTokenCredential(config).GetAccessToken();
            var apiContext = new APIContext(accessToken);
            return apiContext;
        }

        private static void ListPaymentsDemo()
        {
            var apiContext = GetApiContext();

            var history = Payment.List(apiContext);

            foreach (var payment in history.payments)
            {
                PrintPayment(payment);
            }
        }

        private static void PrintPayment(Payment payment)
        {
            var output = string.Format(@"Payment from:{0} {1} state:{3} amount:{4}{5} id{6} PayerId:{7}",
                payment.payer.payer_info.first_name,
                payment.payer.payer_info.last_name,
                payment.state,
                payment.transactions[0].amount.total,
                payment.transactions[0].amount.currency,
                payment.id,
                payment.payer.payer_info.payer_id
                );

            Console.WriteLine(output);
        }

        private static void ExecutePaymentDemo()
        {
            // At this point buyer was redirected to approval_url in browser and approved the payment

            // Buyer's browser is redirected to http://ekjov.dyn.pingec.si:8888/mobileclient.aspx?h_actkey=paypal_return&paymentId=PAY-6Y150393AP3719109LIO5JCY&token=EC-5U115267N62523746&PayerID=XRHV52C9VFUDQ

            // Execute the payment
            var paymentId = "PAY-6Y150393AP3719109LIO5JCY";
            var payerId = "XRHV52C9VFUDQ";

            var apiContext = GetApiContext();

            Payment payment = Payment.Get(apiContext, paymentId);
            var paymentExecution = new PaymentExecution
            {
                payer_id = payerId
            };

            payment.Execute(apiContext, paymentExecution);

            // Check payment.state, possible options are:
            //created
            //approved
            //failed
            //canceled
            //expired

            // The following should be true: payment.state == "approved"
            // If not, payment can be refreshed with Payment.Get(apiContext, "PAY-6Y150393AP3719109LIO5JCY"); in order to recheck state
        }

        private static void CreatePaymentDemo()
        {
            // Authenticate with PayPal
            var apiContext = GetApiContext();

            // Make an API call
            var payment = Payment.Create(apiContext, new Payment
            {
                intent = "sale",
                payer = new Payer
                {
                    payment_method = "paypal"
                },
                transactions = new List<Transaction>
                {
                    new Transaction
                    {
                        description = "Transaction description.",
                        invoice_number = "001",
                        amount = new Amount
                        {
                            currency = "USD",
                            total = "100.00",
                            details = new Details
                            {
                                tax = "15",
                                shipping = "10",
                                subtotal = "75"
                            }
                        },
                        item_list = new ItemList
                        {
                            items = new List<Item>
                            {
                                new Item
                                {
                                    name = "Item Name",
                                    currency = "USD",
                                    price = "15",
                                    quantity = "5",
                                    sku = "sku"
                                }
                            }
                        }
                    }
                },
                redirect_urls = new RedirectUrls
                {
                    return_url = "http://ekjov.dyn.pingec.si:8888/mobileclient.aspx?h_actkey=paypal_return",
                    cancel_url = "http://ekjov.dyn.pingec.si:8888/mobileclient.aspx?h_actkey=paypal_cancel"
                }
            });

            // Get approval_url for buyer

            var approval_url_for_buyer = "";
            var links = payment.links.GetEnumerator();
            while (links.MoveNext())
            {
                var link = links.Current;
                if (link.rel.ToLower().Trim().Equals("approval_url"))
                {
                    approval_url_for_buyer = link.href;
                }
            }

            // Redirect buyer's browser to approval_url to authorize the payment
        }
    }
}