本人不是专门的程序员,所以代码质量我自己也知道很渣,纯属为了跑通功能写的.

1. JSAPI使用了腾讯支付的V3api, 要求证书加密解密的,之前用过PHP版本跑通过V2的,原本以为是件很简单的事情,发现自己低估了难度.
2. 走了个弯路,对腾讯支付统一下单的流程自己误解了先后顺序,以为是首先调用腾讯的jsapi唤起微信来做支付,然后支付过程是统一下单,在这个问题上纠结了一个周末. 突然发现自己完全理解错了,是先要通过统一下单api生成prepay_id 后,再调用jsapi来唤起微信支付.
3. 测试环境没有部署在公网,直接写hosts方式来,所以notify是收不到的,这个也很容易理解.目前自己也就跑到调用其支付后面暂时没写了,等后面再来补充.
4. 因为使用的asp.net core mvc 模式,再调用jsapi总是报签名验证错误,但自己点断抓生成的签名和使用腾讯官方提供的签名验证工具校验是一直, 这个昨天折腾了一下午,最后发现是view视图编译后, 对页面上的paySign进行了转义,将+进行了转义,这是个大坑,而且很难发现,最后用@HTML.Raw 强制显示原文.
5. 回顾整个撸码的过程,虽然腾讯官方没有提供v3 的sdk .net 的版本, 但一些核心关键代码是已经给出来了,比如签名过程.

这个是核心的签名过程的类,商户ID和证书序号在支付后台都可以获取

    public class HttpHandler : DelegatingHandler
    {
        private readonly string merchantId;   \\商户ID
        private readonly string serialNo;   \\证书序号
        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);
            //官网的代码没有添加 User-Agent 和  Accept  导致一直报错, 但是api文档里面又提到需要.
            request.Headers.Add("User-Agent", "Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/119.0.0.0 Mobile Safari/537.36 Edg/119.0.0.0");
            request.Headers.Add("Accept", "application/json");
            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();
            }
            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 = WxPayTool.Sign(message);
                //Sign(message);
            return $"mchid=\"{merchantId}\",nonce_str=\"{nonce}\",timestamp=\"{timestamp}\",serial_no=\"{serialNo}\",signature=\"{signature}\"";
        }
        protected string Sign(string message)
        {
            //   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 = @"";   //最好是用notepad去打开你腾讯支付后台获取到  apiclient_key.pem, 正式代码可以写个类去读私钥文件的内容,私钥存在指定位置.
            byte[] keyData = Convert.FromBase64String(privateKey);

            var rsa = RSA.Create();
            //适用该方法的版本https://learn.microsoft.com/zh-cn/dotnet/api/system.security.cryptography.asymmetricalgorithm.importpkcs8privatekey?view=net-7.0
            rsa.ImportPkcs8PrivateKey(keyData, out _);
            byte[] data = System.Text.Encoding.UTF8.GetBytes(message);
            return Convert.ToBase64String(rsa.SignData(data, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1));
        }

控制器执行统一下单,并构造jaspi所需要的数据包

public  IActionResult  pay() {
    string prepay_id = "";
    OrderData orderData = new OrderData();    
    orderData.appid = "";    //企业微信ID
    orderData.mchid = "";   //商户ID
    orderData.description = "";
    orderData.out_trade_no = "DF-" + DateTime.Now.ToString("yyyyMMddHHmmssMM");
    orderData.notify_url = "";
    orderData.attach = "";
    orderData.amount = new OrderData.WxPayAmountModel
    {
        total = 1,
        currency = "CNY"
    };
    orderData.payer = new OrderData.Payer
    {
        openid = "oKpVN05g3KNRBTWGbPOnzw8Z64Mg"
    };
    string url = "https://api.mch.weixin.qq.com/v3/pay/transactions/jsapi";
    HttpClient client = new HttpClient(new HttpHandler(" #商户ID# ", " #商户证书序号#"));
    var bodyJson  = new   StringContent(JsonConvert.SerializeObject(orderData).ToString(), Encoding.UTF8, "application/json");
    var response = client.PostAsync(url, bodyJson);
    var rep = response.Result;//在这里会等待task返回
    var respStr = rep.Content.ReadAsStringAsync().Result;
     JObject jo = (JObject)JsonConvert.DeserializeObject(respStr);
    prepay_id = jo["prepay_id"].ToString();
    WxPrePayData prePayData = new WxPrePayData();
    prePayData.PrePayBillId = prepay_id;
    prePayData.NonceStr = Path.GetRandomFileName();

    prePayData.TimeStamp = DateTimeOffset.Now.ToUnixTimeSeconds().ToString();
    prePayData.PrePayAmount = 1;
    prePayData.AppId = "#企业微信ID#";
    prePayData.Package = "prepay_id=" + prePayData.PrePayBillId;   //这个prepay_id 是统一下单接口返回来的,这里我也折腾了好就才搞明白
    string message = prePayData.AppId + "\n" + prePayData.TimeStamp + "\n" + prePayData.NonceStr + "\n" + prePayData.Package + "\n";
    prePayData.PaySign = WxPayTool.Sign(message);

    ViewData["data"] = prePayData;

      return  View();
}

public class OrderData
{
    public string appid {  get; set; }
    public string mchid { get; set; }
    public string description { get; set; }

    public string out_trade_no { get; set; }

    public string notify_url { get; set; }

    public string attach { set; get; }

    public WxPayAmountModel amount { set; get; }
    public Payer payer { set; get; }    

    public class WxPayAmountModel
    {
        /// <summary>
        /// 订单总金额，单位为分。
        /// </summary>
        public int total { set; get; }

        /// <summary>
        /// 货币类型,CNY：人民币，境内商户号仅支持人民币。
        /// </summary>
        public string currency { set; get; } = "CNY";
    }

    public class Payer { 
    
        public string openid { set; get; }
    }
}

public class WxPrePayData
{
     public string  AppId { get; set; }
    public string TimeStamp { get; set; }
    public string NonceStr { get; set; }
    public string Package { get; set; }
    public string PaySign { get; set;}
    public static string signType = "RSA";

}

支付前端核心js代码直接用的官网上的实例, 我这里是点击id=“pay” 的按钮调用jsapi支付,加载页面前已经完成了统一下单过程.

    $("#pay").click(function () {

        function onBridgeReady() {
            WeixinJSBridge.invoke('getBrandWCPayRequest', {
                "appId": "@data.AppId",     //公众号ID，由商户传入
                "timeStamp": "@data.TimeStamp",     //时间戳，自1970年以来的秒数
                "nonceStr": "@data.NonceStr",      //随机串
                "package": "@data.Package",
                "signType": "RSA",     //微信签名方式：
                "paySign": "@Html.Raw(data.PaySign)"  //微信签名
            },
                function (res) {
                    alert(res.err_msg);
                    if (res.err_msg == "get_brand_wcpay_request:ok") {
                        // 使用以上方式判断前端返回,微信团队郑重提示：
                        //res.err_msg将在用户支付成功后返回ok，但并不保证它绝对可靠。
                    }
                });
        }
        if (typeof WeixinJSBridge == "undefined") {
            if (document.addEventListener) {
                document.addEventListener('WeixinJSBridgeReady', onBridgeReady, false);
            } else if (document.attachEvent) {
                document.attachEvent('WeixinJSBridgeReady', onBridgeReady);
                document.attachEvent('onWeixinJSBridgeReady', onBridgeReady);
            }
        } else {
            onBridgeReady();
        }


    })

另外就是控制器需要和你在支付后台设置的jsapi的目录保持一致的.