using System;
using System.IO;
using System.Net.Http;
using System.Security.Cryptography;
using System.Threading;
using System.Threading.Tasks;

namespace HttpHandlerDemo
{
    // 使用方法
    // HttpClient client = new HttpClient(new HttpHandler("{商户号}", "{商户证书序列号}"));
    // ...
    // var response = client.GetAsync("https://api.mch.weixin.qq.com/v3/certificates");
    public class HttpHandler : DelegatingHandler
    {
        private readonly string merchantId;
        private readonly string serialNo;

        public HttpHandler(string merchantId, string merchantSerialNo)
        {
            InnerHandler = new HttpClientHandler();

            this.merchantId = merchantId;
            this.serialNo = merchantSerialNo;
        }

        protected async override Task<HttpResponseMessage> SendAsync(
            HttpRequestMessage request,
            CancellationToken cancellationToken)
        {
            var auth = await BuildAuthAsync(request);
            string value = $"WECHATPAY2-SHA256-RSA2048 {auth}";
            request.Headers.Add("Authorization", value);
            request.Headers.Add("Accept", "application/json");//如果缺少这句代码就会导致下单接口请求失败，报400错误（Bad Request）
            request.Headers.Add("User-Agent", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)");//如果缺少这句代码就会导致下单接口请求失败，报400错误（Bad Request）

            return await base.SendAsync(request, cancellationToken);
        }

        protected async Task<string> BuildAuthAsync(HttpRequestMessage request)
        {
            string method = request.Method.ToString();
            string body = "";
            if (method == "POST" || method == "PUT" || method == "PATCH")
            {
                var content = request.Content;
                body = await content.ReadAsStringAsync();//debug的时候在这里打个断点，看看body的值是多少，如果跟你传入的参数不一致，说明是有问题的，一定参考我的方法
            }

            string uri = request.RequestUri.PathAndQuery;
            var timestamp = DateTimeOffset.Now.ToUnixTimeSeconds();
            string nonce = Path.GetRandomFileName();

            string message = $"{method}\n{uri}\n{timestamp}\n{nonce}\n{body}\n";
            string signature = Sign(message);
            return $"mchid=\"{merchantId}\",nonce_str=\"{nonce}\",timestamp=\"{timestamp}\",serial_no=\"{serialNo}\",signature=\"{signature}\"";
        }

        protected string Sign(string message)
        {
            // NOTE： 私钥不包括私钥文件起始的-----BEGIN PRIVATE KEY-----
            //        亦不包括结尾的-----END PRIVATE KEY-----
            string privateKey = "{你的私钥}";
            byte[] keyData = Convert.FromBase64String(privateKey);
            using (CngKey cngKey = CngKey.Import(keyData, CngKeyBlobFormat.Pkcs8PrivateBlob))
            using (RSACng rsa = new RSACng(cngKey))
            {
                byte[] data = System.Text.Encoding.UTF8.GetBytes(message);
                return Convert.ToBase64String(rsa.SignData(data, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
            }
        }
    }
}

 var url = "https://api.mch.weixin.qq.com/v3/pay/transactions/app";
 var req = new GenerateOrderModelForWxPay
                {
                    appid = WxPayConst.appid,
                    mchid = WxPayConst.mchid,
                    description = "商品名称",
                    amount = new WxPayAmountModel
                    {
                        total = 1
                    },
                    out_trade_no = orderNumber,
                    notify_url = "https://xxx.com/api/WxPayCallback"
                };
HttpClient client = new HttpClient(new HttpHandler("{商户号}", "{商户证书序列号}"));
//GET 方式
var response = client.GetAsync("https://api.mch.weixin.qq.com/v3/certificates");

// POST 方式
 var bodyJson = new StringContent(req.ToJson(), Encoding.UTF8, "application/json"); //一定要这样传递参数，不然在加密签名的时候获取到的参数就是\\u0这种形式的数据了，不是传递的这样的数据了，导致加密的结果不正确
var response = await client.PostAsync(url, bodyJson);

// 读取统一下单之后的返回结果，这样读取出来的直接就是结果，或者错误原因，大家一定要这么搞啊！！！多么痛的领悟，会有具体的错误信息的。
var respStr = await response.Content.ReadAsStringAsync();//这里面就包含prepay_id了