services.AddMvc().AddNewtonsoftJson();

或者：

services.AddMvc().AddNewtonsoftJson(options =>options.SerializerSettings.ContractResolver =new CamelCasePropertyNamesContractResolver());

using System;
using PayPalCheckoutSdk.Core;using System.IO;
using System.Text;
using System.Runtime.Serialization.Json;namespace PayPal
{public class PayPalClient{// Place these static properties into a settings area.public static string SandboxClientId { get; set; } = "<alert>{PayPal SANDBOX Client Id}</alert>";public static string SandboxClientSecret { get; set; } = "<alert>{PayPal SANDBOX Client Secret}</alert>";public static string LiveClientId { get; set; } = "<alert>{PayPal LIVE Client Id}</alert>";public static string LiveClientSecret { get; set; } = "<alert>{PayPal LIVE Client Secret}</alert>";///<summary>/// Set up PayPal environment with sandbox credentials./// In production, use LiveEnvironment.///</summary>public static PayPalEnvironment Environment(){
#if DEBUG// You may want to create a UAT (user exceptance tester) // role and check for this:// "if(_unitOfWork.IsUATTester(GetUserId())" instead of fcomiler directives.return new SandboxEnvironment(<alert>SandboxClientId</alert>,<alert>SandboxClientSecret</alert>);
#elsereturn new LiveEnvironment(<alert>LiveClientId</alert>, <alert>LiveClientSecret</alert>);
#endif}///<summary>/// Returns PayPalHttpClient instance to invoke PayPal APIs.///</summary>public static PayPalCheckoutSdk.Core.PayPalHttpClient Client(){return new PayPalHttpClient(Environment());}public static PayPalCheckoutSdk.Core.PayPalHttpClient Client(string refreshToken){return new PayPalHttpClient(Environment(), refreshToken);}///<summary>/// Use this method to serialize Object to a JSON string.///</summary>public static String ObjectToJSONString(Object serializableObject){MemoryStream memoryStream = new MemoryStream();var writer = JsonReaderWriterFactory.CreateJsonWriter(memoryStream,Encoding.UTF8,true,true,"  ");var ser = new DataContractJsonSerializer(serializableObject.GetType(),new DataContractJsonSerializerSettings {UseSimpleDictionaryFormat = true });ser.WriteObject(writer,serializableObject);memoryStream.Position = 0;StreamReader sr = new StreamReader(memoryStream);return sr.ReadToEnd();}}
}

using System.Net;
using System.Net.Http.Headers;namespace PayPal
{public class SmartButtonHttpResponse{readonly PayPalCheckoutSdk.Orders.Order _result;public SmartButtonHttpResponse(PayPalHttp.HttpResponse httpResponse){Headers = httpResponse.Headers;StatusCode = httpResponse.StatusCode;_result = httpResponse.Result<PayPalCheckoutSdk.Orders.Order>();}public HttpHeaders Headers { get; }public HttpStatusCode StatusCode { get; }public PayPalCheckoutSdk.Orders.Order Result(){return _result;}public string orderID { get; set; }}
}

using PayPalCheckoutSdk.Orders;
using System.Collections.Generic;namespace PayPal
{public static class OrderBuilder{/// <summary>/// Use classes from the PayPalCheckoutSdk to build an OrderRequest/// </summary>/// <returns></returns>public static OrderRequest Build(){OrderRequest orderRequest = new OrderRequest();// Add code to fill out the order request properties<alert>// See the attached source code for a more detailed example.</alert> return orderRequest;}}
}

using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;using PayPalCheckoutSdk.Orders;namespace Test.Controllers
{public class CheckoutController : Controller{/// <summary>/// Action to display the cart form for the SERVER side integration/// </summary>/// <returns></returns>public IActionResult Index(){
#if DEBUG// You may want to create a UAT (user exceptance tester) role // and check for this:// "if(_unitOfWork.IsUATTester(GetUserId())"// Company SANDBOX Client Id. To go live replace this with the live ID.ViewBag.ClientId = <alert>PayPal.PayPalClient.SandboxClientId</alert>; // Get from a // data store or stettings
#else// Company LIVE Client Id. To go live replace this with the live ID.ViewBag.ClientId = <alert>PayPal.PayPalClient.LiveClientId</alert>; // Get from a // data store or stettings
#endifViewBag.CurrencyCode = "GBP"; // Get from a data storeViewBag.CurrencySign = "£";   // Get from a data storereturn View();}/// <summary>/// This action is called when the user clicks on the PayPal button./// </summary>/// <returns></returns>[Route("api/paypal/checkout/order/create")]public async Task<PayPal.SmartButtonHttpResponse> Create(){var request = new PayPalCheckoutSdk.Orders.OrdersCreateRequest();request.Prefer("return=representation");request.RequestBody(PayPal.OrderBuilder.Build());// Call PayPal to set up a transactionvar response = await PayPal.PayPalClient.Client().Execute(request);// Create a response, with an order id.var result = response.Result<PayPalCheckoutSdk.Orders.Order>();var payPalHttpResponse = new PayPal.SmartButtonHttpResponse(response){orderID = result.Id};return payPalHttpResponse;}/// <summary>/// This action is called once the PayPal transaction is approved/// </summary>/// <param name="orderId"></param>/// <returns></returns>[Route("api/paypal/checkout/order/approved/{orderId}")]public IActionResult Approved(string orderId){return Ok();}/// <summary>/// This action is called once the PayPal transaction is complete/// </summary>/// <param name="orderId"></param>/// <returns></returns>[Route("api/paypal/checkout/order/complete/{orderId}")]public IActionResult Complete(string orderId){// 1. Update the database.// 2. Complete the order process. Create and send invoices etc.// 3. Complete the shipping process.return Ok();}/// <summary>/// This action is called once the PayPal transaction is complete/// </summary>/// <param name="orderId"></param>/// <returns></returns>[Route("api/paypal/checkout/order/cancel/{orderId}")]public IActionResult Cancel(string orderId){// 1. Remove the orderId from the database.return Ok();}/// <summary>/// This action is called once the PayPal transaction is complete/// </summary>/// <param name="orderId"></param>/// <returns></returns>[Route("api/paypal/checkout/order/error/{orderId}/{error}")]public IActionResult Error(string orderId,string error){// Log the error.// Notify the user.return NoContent();}}
}

<!-- Set up a container element for the PayPal smart button -->
<div id="paypal-button-container"></div><!-- Include the PayPal JavaScript SDK -->
<script src="https://www.paypal.com/sdk/js?client-id=@ViewBag.ClientId&currency=@ViewBag.CurrencyCode"></script><script>// This is stored just in case the user cancels the other // or there is an error in the other process.var orderId;// Render the PayPal smart button into #paypal-button-containerpaypal.Buttons({// Set up the transactioncreateOrder: function (data, actions) {orderId = data.orderID;return fetch('/api/paypal/checkout/order/create/', {method: 'post'}).then(function (res) {return res.json();}).then(function (data) {return data.orderID;});},// Finalise the transactiononApprove: function (data, actions) {return fetch('/api/paypal/checkout/order/approved/' + data.orderID, {method: 'post'}).then(function (res) {return actions.order.capture();}).then(function (details) {// (Preferred) Notify the server that the transaction id complete // and have an option to display an order completed screen.window.location.replace('/api/paypal/checkout/order/complete/' + data.orderID + '/@ViewBag.CurrencyCode');// OR// Notify the server that the transaction id complete//httpGet('/api/paypal/checkout/order/complete/' + data.orderID);// Show a success message to the buyeralert('Transaction completed by ' + details.payer.name.given_name + '!');});},// Buyer cancelled the paymentonCancel: function (data, actions) {httpGet('/api/paypal/checkout/order/cancel/' + data.orderID);},// An error occurred during the transactiononError: function (err) {httpGet('/api/paypal/checkout/order/error/' + orderId + '/' + encodeURIComponent(err));}}).render('#paypal-button-container');
</script>

除了URL，此代码对于所有解决方案都是相同的。

function httpGet(url) {var xmlHttp = new XMLHttpRequest();xmlHttp.open("GET", url, false);xmlHttp.send(null);return xmlHttp.responseText;
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;namespace PayPal.Values
{/// <summary>/// The intent to either capture payment immediately or/// authorize a payment for an order after order creation./// </summary>public static class CheckoutPaymentIntent{/// <summary>/// The merchant intends to capture payment immediately after /// the customer makes a payment./// </summary>public static string CAPTURE { get; private set; } = "CAPTURE";/// <summary>/// The merchant intends to authorize a payment and/// place funds on hold after the customer makes a payment./// Authorized payments are guaranteed for up to three days but/// are available to capture for up to 29 days./// After the three-day honor period, the original authorized payment expires/// and you must re-authorize the payment./// You must make a separate request to capture payments on demand./// This intent is not supported when you have more than one `purchase_unit` /// within your order./// </summary>public static string AUTHORIZE { get; private set; } = "AUTHORIZE";}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;namespace PayPal.Values
{public static class CurrencyCode{/// <summary>/// Great British Pounds/// </summary>public static string GBP { get; private set; } = "GBP";/// <summary>/// US Dolars/// </summary>public static string USD { get; private set; } = "USD";/// <summary>/// Euros/// </summary>public static string EUR { get; private set; } = "EUR";}
}

根据需要添加其他货币。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;namespace PayPal.Values
{/// <summary>/// The type of landing page to show on the PayPal site for customer checkout./// Default: NO_PREFERENCE./// Source: https://developer.paypal.com/docs/api/orders/v2//// </summary>public class LandingPage{/// <summary>/// When the customer clicks PayPal Checkout, /// the customer is redirected to a page to log in to PayPal /// and approve the payment./// </summary>public static string LOGIN { get; private set; } = "LOGIN";/// <summary>/// When the customer clicks PayPal Checkout, /// the customer is redirected to a page to enter credit or /// debit card and other relevant billing information required to /// complete the purchase./// </summary>public static string BILLING { get; private set; } = "BILLING";/// <summary>/// When the customer clicks PayPal Checkout,/// the customer is redirected to either a page to log in to PayPal and/// approve the payment or to a page to enter credit or/// debit card and other relevant billing information/// required to complete the purchase, depending on their /// previous interaction with PayPal./// </summary>public static string NO_PREFERENCE { get; private set; } = "NO_PREFERENCE";}
}

namespace PayPal.Values
{/// <summary>/// The shipping preference:////// * Displays the shipping address to the customer./// * Enables the customer to choose an address on the PayPal site./// * Restricts the customer from changing the address ///   during the payment-approval process.////// Default: GET_FROM_FILE./// Source: https://developer.paypal.com/docs/api/orders/v2//// </summary>public static class ShippingPreference{/// <summary>/// Use the customer-provided shipping address on the PayPal site./// </summary>public static string GET_FROM_FILE { get; private set; } = "GET_FROM_FILE";/// <summary>/// Redact the shipping address from the PayPal site. /// Recommended for digital goods./// </summary>public static string NO_SHIPPING { get; private set; } = "NO_SHIPPING";/// <summary>/// Use the merchant-provided address. /// The customer cannot change this address on the PayPal site./// </summary>public static string SET_PROVIDED_ADDRESS { get; private set; } = "SET_PROVIDED_ADDRESS";}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;namespace PayPal.Values
{/// <summary>/// Configures a Continue or Pay Now checkout flow./// Source: https://developer.paypal.com/docs/api/orders/v2//// </summary>public static class UserAction{/// <summary>/// After you redirect the customer to the PayPal payment page,/// a Continue button appears. Use this option when the final amount is not known/// when the checkout flow is initiated and you want to redirect/// the customer to the merchant page without processing the payment./// </summary>public static string CONTINUE { get; private set; } = "CONTINUE";/// <summary>/// After you redirect the customer to the PayPal payment page,/// a Pay Now button appears./// Use this option when the final amount is known when the checkout is initiated/// and you want to process the payment immediately /// when the customer clicks Pay Now./// </summary>public static string PAY_NOW { get; private set; } = "PAY_NOW";}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;namespace PayPal.Values.Item
{/// <summary>/// The item category type./// </summary>public static class Category{/// <summary>/// Goods that are stored, delivered, and/// used in their electronic format./// This value is not currently supported for API callers that leverage the/// [PayPal for Commerce Platform]/// (https://www.paypal.com/us/webapps/mpp/commerce-platform) product./// </summary>public static string DIGITAL_GOODS { get; private set; } = "DIGITAL_GOODS";/// <summary>/// A tangible item that can be shipped with proof of delivery./// </summary>public static string PHYSICAL_GOODS { get; private set; } = "PHYSICAL_GOODS";}
}