1.https://developer.paypal.com/ 注册账号，并且申请一个app，获得 client id，secret等数据
2.点击页面中"Sandbox Account"按钮，创建至少两个账号(一个收钱用，一个付钱用)，paypal中sandbox是测试环境。live是上线后的环境。
3.下载paypal sdk for .net，https://github.com/paypal/rest-api-sdk-dotnet
4.新建一个解决方案(本例为mvc4 web程序:Paypalsample)，根据需求选择适合的SDK文件,或者还可以通过NuGet(如果你的vs带有此功能)的方式来添加SDK，以下是vs2012的添加过程
5.OK，paypal SDK加进来了，接下来要对web.config文件进行配置,参考地址：https://github.com/paypal/sdk-core-dotnet/wiki/SDK-Configuration-Parameters#list-of-configuration-parameters
<configSections>
<!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
<section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
<section name="paypal" type="PayPal.Manager.SDKConfigHandler, PayPalCoreSDK"/>
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net"/>
</configSections>
<!-- PayPal SDK config -->
<paypal>
<settings>
<!--<add name="endpoint" value="https://api.sandbox.paypal.com"/>-->
<add name="mode" value="sandbox" /><!--or set value "live" -->
<add name="connectionTimeout" value="360000"/>
<add name="requestRetries" value="1"/>
<add name="ClientID" value="AU0OwhDpDM05PVDfyhm3qpgdXfIc0hNv2O_HaaXbHhA_BvZgwD6sbVloijMW"/>
<add name="ClientSecret" value="EM3wHxDSvX3QTRKxPMZKHmBtjxYQtscVzzimbnD7FWx7aE0kJ6H5srmvtyZ0"/>
</settings>
</paypal>
<log4net>
<appender name="FileAppender" type="log4net.Appender.FileAppender">
<file value="RestApiSDKUnitTestLog.log"/>
<appendToFile value="true"/>
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date [%thread] %-5level %logger [%property{NDC}] %message%newline"/>
</layout>
</appender>
<root>
<level value="DEBUG"/>
<appender-ref ref="FileAppender"/>
</root>
</log4net>
6.接下来就是调用Paypal API实现支付，上文中提到下载SDK，其中有sample的例子，可以参考看看，里面实现了大部分常用的api，本例只实现支付的功能。
a.(视图)页面1部分:
@{ ViewBag.Title = "Index"; } <a href="@Url.Action("Payment")">Payment with Paypal account</a>
b.页面2部分：
@{ ViewBag.Title = "Payment"; } <h1>Payment Status: @ViewBag.Status</h1>
c.(controller)页面2后台代码部分：
public ActionResult Payment() { //get access token string accessToken = null; string clientID = ConfigManager.Instance.GetProperties()["ClientID"]; string clientSecret = ConfigManager.Instance.GetProperties()["ClientSecret"]; OAuthTokenCredential TokenCredential = new OAuthTokenCredential(clientID, clientSecret); accessToken = TokenCredential.GetAccessToken(); Payment pymnt = null; // ## ExecutePayment if (Request.Params["PayerID"] != null) { pymnt = new Payment(); if (Request.Params["guid"] != null) { pymnt.id = (string)Session[Request.Params["guid"]]; try { PaymentExecution pymntExecution = new PaymentExecution(); pymntExecution.payer_id = Request.Params["PayerID"]; Payment executedPayment = pymnt.Execute(accessToken, pymntExecution); ViewBag.Status = executedPayment.state; } catch (PayPal.Exception.PayPalException ex) { } } } // ## Creating Payment else { // ###Items // Items within a transaction. Item item = new Item(); item.name = "笔记本电脑"; item.currency = "USD"; item.price = "1000"; item.quantity = "5"; item.sku = "台"; List<Item> itms = new List<Item>(); itms.Add(item); ItemList itemList = new ItemList(); itemList.items = itms; // ###Payer // A resource representing a Payer that funds a payment // Payment Method // as `paypal` Payer payr = new Payer(); payr.payment_method = "paypal"; Random rndm = new Random(); var guid = Convert.ToString(rndm.Next(100000)); string baseURI = Request.Url.Scheme + "://" + Request.Url.Authority + "/Paypal/"; // # Redirect URLS RedirectUrls redirUrls = new RedirectUrls(); redirUrls.cancel_url = baseURI + "guid=" + guid; redirUrls.return_url = baseURI + "Payment?guid=" + guid; // ###Details // Let's you specify details of a payment amount. Details details = new Details(); details.tax = "10"; details.shipping = "10"; details.subtotal = "5000"; // ###Amount // Let's you specify a payment amount. Amount amnt = new Amount(); amnt.currency = "USD"; // Total must be equal to sum of shipping, tax and subtotal. amnt.total = "5020"; amnt.details = details; // ###Transaction // A transaction defines the contract of a // payment - what is the payment for and who // is fulfilling it. List<Transaction> transactionList = new List<Transaction>(); Transaction tran = new Transaction(); tran.description = "Transaction description."; tran.amount = amnt; tran.item_list = itemList; // The Payment creation API requires a list of // Transaction; add the created `Transaction` // to a List transactionList.Add(tran); // ###Payment // A Payment Resource; create one using // the above types and intent as `sale` or `authorize` pymnt = new Payment(); pymnt.intent = "sale"; pymnt.payer = payr; pymnt.transactions = transactionList; pymnt.redirect_urls = redirUrls; try { string approvalUrl = ""; Payment createdPayment = pymnt.Create(accessToken); var links = createdPayment.links.GetEnumerator(); while (links.MoveNext()) { Links lnk = links.Current; if (lnk.rel.ToLower().Trim().Equals("approval_url")) { approvalUrl = lnk.href; } } Session.Add(guid, createdPayment.id); if (!string.IsNullOrEmpty(approvalUrl)) return Redirect(approvalUrl); } catch (PayPal.Exception.PayPalException ex) { } } return View(); }
c.注意return_url的正确性，总钱数和分项目的钱数要对到，total=subtotal+shipping+tax.其中有些字段非必须，详细参考官方api：https://developer.paypal.com/webapps/developer/docs/api/#create-a-payment
7.运行调试
付款成功后payment status字段的值会有所变化，此支付方式分成两个步骤，第一是create payment，创建一个支付清单，然后用户输入用户名和密码进行支付确认，然后执行ExecutePayment方法进行支付。支付成功后再去看看付款的账号的余额和收款账号的余额。
以同样的方式去查看收款账号是否收到钱，整个过程结束。