麻烦用C# .net写

好的，下面是使用C# .NET Framework写的微信扫码支付的WEB应用程序示例代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Xml;
using System.Xml.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Net;
using System.IO;

namespace WeChatPayDemo.Controllers
{
    public class HomeController : Controller
    {
        // 商户号
        const string mch_id = "your-mch-id";
        // 商户API密钥
        const string api_key = "your-api-key";
        // 应用ID
        const string appid = "your-appid";
        // 通知地址
        const string notify_url = "http://your-domain.com/notify";
        // 支付请求地址
        const string pay_url = "https://api.mch.weixin.qq.com/pay/unifiedorder";

        // 首页
        public ActionResult Index()
        {
            return View();
        }

        // 支付页面
        public ActionResult Pay(string total_fee)
        {
            // 生成订单号
            string out_trade_no = DateTime.Now.ToString("yyyyMMddHHmmssfff") + new Random().Next(1000, 9999);
            // 构造支付请求参数
            SortedDictionary<string, object> parameters = new SortedDictionary<string, object>();
            parameters.Add("appid", appid);
            parameters.Add("mch_id", mch_id);
            parameters.Add("nonce_str", Guid.NewGuid().ToString().Replace("-", ""));
            parameters.Add("body", "微信扫码支付");
            parameters.Add("out_trade_no", out_trade_no);
            parameters.Add("total_fee", int.Parse(float.Parse(total_fee) * 100 + ""));
            parameters.Add("spbill_create_ip", Request.UserHostAddress);
            parameters.Add("notify_url", notify_url);
            parameters.Add("trade_type", "NATIVE");

            // 签名
            string sign = Sign(parameters);
            parameters.Add("sign", sign);

            // 构造XML请求内容
            string requestXml = BuildXml(parameters);

            // 发送支付请求
            string responseXml = HttpPost(pay_url, requestXml);

            // 解析响应XML
            XDocument doc = XDocument.Parse(responseXml);
            string prepay_id = doc.Element("xml").Element("prepay_id").Value;
            string code_url = doc.Element("xml").Element("code_url").Value;

            // 构造二维码图片链接
            string qr_code_url = $"https://api.qrserver.com/v1/create-qr-code/?size=150x150&data={HttpUtility.UrlEncode(code_url)}";

            // 传递二维码图片链接和订单号到支付页面
            ViewBag.QRCodeUrl = qr_code_url;
            ViewBag.OutTradeNo = out_trade_no;

            return View();
        }

        // 支付结果通知接口
        public ActionResult Notify()
        {
            // 接收POST数据
            Stream stream = Request.InputStream;
            byte[] bytes = new byte[stream.Length];
            stream.Read(bytes, 0, bytes.Length);
            string xml = Encoding.UTF8.GetString(bytes);

            // 解析XML数据
            XDocument doc = XDocument.Parse(xml);
            string return_code = doc.Element("xml").Element("return_code").Value;
            string return_msg = doc.Element("xml").Element("return_msg").Value;
            string result_code = doc.Element("xml").Element("result_code").Value;
            string out_trade_no = doc.Element("xml").Element("out_trade_no").Value;
            string transaction_id = doc.Element("xml").Element("transaction_id").Value;

            // 验证签名
            SortedDictionary<string, object> parameters = new SortedDictionary<string, object>();
            parameters.Add("return_code", return_code);
            parameters.Add("return_msg", return_msg);
            parameters.Add("result_code", result_code);
            parameters.Add("out_trade_no", out_trade_no);
            parameters.Add("transaction_id", transaction_id);

            string sign = Sign(parameters);
            string sign_received = doc.Element("xml").Element("sign").Value;

            if (sign == sign_received)
            {
                // 处理支付结果
                // ...
                // 返回通知结果
                return Content(BuildXml(new Dictionary<string, object> { { "return_code", "SUCCESS" }, { "return_msg", "OK" } }));
            }
            else
            {
                // 签名验证失败
                // ...
                // 返回错误信息
                return Content(BuildXml(new Dictionary<string, object> { { "return_code", "FAIL" }, { "return_msg", "签名验证失败" } }));
            }
        }

        // 签名算法
        private string Sign(IDictionary<string, object> parameters)
        {
            string stringA = string.Join("&", parameters.Where(p => !string.IsNullOrEmpty(p.Value.ToString())).OrderBy(p => p.Key).Select(p => $"{p.Key}={p.Value}"));
            string stringSignTemp = $"{stringA}&key={api_key}";
            string sign = MD5(stringSignTemp).ToUpper();
            return sign;
        }

        // MD5算法
        private string MD5(string input)
        {
            MD5 md5 = MD5.Create();
            byte[] buffer = Encoding.UTF8.GetBytes(input);
            byte[] hash = md5.ComputeHash(buffer);
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < hash.Length; i++)
            {
                sb.Append(hash[i].ToString("x2"));
            }
            return sb.ToString();
        }

        // 构造XML请求内容
        private string BuildXml(IDictionary<string, object> parameters)
        {
            StringBuilder sb = new StringBuilder();
            sb.Append("<xml>");
            foreach (var kv in parameters)
            {
                sb.Append($"<{kv.Key}>{kv.Value}</{kv.Key}>");
            }
            sb.Append("</xml>");
            return sb.ToString();
        }

        // 发送HTTP POST请求
        private string HttpPost(string url, string data)
        {
            byte[] bytes = Encoding.UTF8.GetBytes(data);
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
            request.Method = "POST";
            request.ContentType = "application/x-www-form-urlencoded";
            request.ContentLength = bytes.Length;
            using (Stream stream = request.GetRequestStream())
            {
                stream.Write(bytes, 0, bytes.Length);
            }
            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            {
                using (Stream stream = response.GetResponseStream())
                {
                    using (StreamReader reader = new StreamReader(stream))
                    {
                        return reader.ReadToEnd();
                    }
                }
            }
        }
    }
}

以上代码仅供参考，实际开发中需要根据实际情况进行相应的修改和完善。同时，也需要遵循相关的安全规范和隐私保护措施，确保用户信息和支付安全。