接口文档：微信支付-开发者文档 (qq.com)

        public const string transactions_url = "https://api.mch.weixin.qq.com/v3/pay/transactions/h5";
        public static string certPath = AppDomain.CurrentDomain.BaseDirectory + @"\cert\apiclient_cert.p12";
        public static string certificatesPath = AppDomain.CurrentDomain.BaseDirectory + @"\cert\certificates_cert.pem"; // 回调解密证书 根据接口获取后 保存为 pem 后缀  我请求后自己手动保存的。没写代码去保存
        private static string certPassword = "888888888"; //.p12 的证书密码
        public string BuildAuth(string mchid, string serial_no, string method, string uri, string body)
        {
        var timestamp = DateTimeOffset.Now.ToUnixTimeSeconds();
        string nonce = Guid.NewGuid().ToString();
        //构造签名串
        //HTTP请求方法\n + URL\n + 请求时间戳\n + 请求随机串\n + 请求报文主体\n
        string message = method + "\n" + uri + "\n" + timestamp + "\n" + nonce + "\n" + body + "\n";
        string signature = GenerateSignature(message);

        //发起请求的商户（包括直连商户、服务商或渠道商）的商户号 mchid
        //商户API证书序列号serial_no，用于声明所使用的证书
        //请求随机串nonce_str
        //时间戳timestamp
        //签名值signature
        return "mchid=\"" + mchid + "\",nonce_str=\"" + nonce + "\",timestamp=\"" + timestamp + "\",serial_no=\"" + serial_no + "\",signature=\"" + signature + "\"";
        }


        /// <summary>
        /// 生成签名
        /// </summary>
        /// <param name="message"></param>
        /// <returns></returns>
        public static string GenerateSignature(string message)
        {
        X509Certificate2 cert = new X509Certificate2(certPath, certPassword, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable);
        RSA PrivateKey = cert.GetRSAPrivateKey();
        byte[] dataBytes = Encoding.UTF8.GetBytes(message);
        byte[] signatureBytes = PrivateKey.SignData(dataBytes, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
        return Convert.ToBase64String(signatureBytes);
        }


        /// <summary>
        /// 验证回调签名
        /// </summary>
        /// <param name="signature"></param>
        /// <param name="timestamp"></param>
        /// <param name="nonce"></param>
        /// <param name="body"></param>
        /// <returns></returns>
        public static bool VerifySignature(string signature, string timestamp, string nonce, string body)
        {
        //应答时间戳\n
        //应答随机串\n
        //应答报文主体\n
        X509Certificate2 wechatCert = new X509Certificate2(certificatesPath); //, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet | X509KeyStorageFlags.Exportable
        string combinedString = string.Format("{0}\n{1}\n{2}\n", timestamp, nonce, body);
        byte[] buff = Encoding.UTF8.GetBytes(combinedString);
        var rsaPar = wechatCert.GetRSAPublicKey().ExportParameters(false);
        var rsa = new RSACryptoServiceProvider();
        rsa.ImportParameters(rsaPar);
        return rsa.VerifyData(buff, CryptoConfig.MapNameToOID("SHA256"), Convert.FromBase64String(signature));
        }


        #region 加密解密

        public static string AesGcmDecrypt(string associatedData, string nonce, string ciphertext, string apikey)
        {
        GcmBlockCipher gcmBlockCipher = new GcmBlockCipher(new AesEngine());
        AeadParameters aeadParameters = new AeadParameters(
        new KeyParameter(Encoding.UTF8.GetBytes(apikey)),
        128,
        Encoding.UTF8.GetBytes(nonce),
        Encoding.UTF8.GetBytes(associatedData));
        gcmBlockCipher.Init(false, aeadParameters);

        byte[] data = Convert.FromBase64String(ciphertext);
        byte[] plaintext = new byte[gcmBlockCipher.GetOutputSize(data.Length)];
        int length = gcmBlockCipher.ProcessBytes(data, 0, data.Length, plaintext, 0);
        gcmBlockCipher.DoFinal(plaintext, length);
        return Encoding.UTF8.GetString(plaintext);
        }
        #endregion
        
        

  遇到的问题有

  1、签名老验证不过去：

  
  生成的签名老验证不过    \n 不要加转义符

  2、 发送的请求老是400  使用工具请求正常。代码不行。
  UserAgent = "m.cnblogs.com/WebRequest"; 
  不要留空就行 网址可填自己的。

  3、回调解密的证书是自己获取的。可以获取后自己保存为后缀 .pem的证书文件
   获取平台证书列表-文档中心-微信支付商户平台 (qq.com)

  4、请求到h5_url 后可以在后面拼接指定跳转链接  https://wx.tenpay.com/cgi-bin/mmpayweb-bin/checkmweb?prepay_id=wx2916263004719461949c84457c735b0000&package=2150917749&redirect_url=System.Web.HttpUtility.UrlEncode("https://www.cnblogs.com/MIMU86")

  5、苹果手机不是默认浏览器会跳到默认浏览器 无解