用的是官方给的.net版demo。
支付能成功。前台也能返回get_brand_wcpay_request:ok
回调页面已经设置过了，外网可以直接访问。
public const string NOTIFY_URL = “http://服务器域名/example/ResultNotifyPage.aspx”;
ResultNotifyPage.aspx后台代码如下

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Text;
using System.Xml.Linq;
using System.IO;
using System.Net;
using DingCanBLL;
public partial class example_ResultNotifyPage : System.Web.UI.Page
{
    OrderBLL oBLL = new OrderBLL();
   
    protected void Page_Load(object sender, EventArgs e)
    {
        string postStr = "";
        /// <summary>
        /// 微信支付回调 
        /// </summary>
        /// <param name="xml"></param>
        if (Request.HttpMethod.ToLower() == "post")
        {
            Stream s = System.Web.HttpContext.Current.Request.InputStream;
            byte[] b = new byte[s.Length];
            s.Read(b, 0, (int)s.Length);
            postStr = Encoding.UTF8.GetString(b);
            if (!string.IsNullOrEmpty(postStr))
            {
                //封装请求类
                var res = XDocument.Parse(postStr);
                if (res.Element("xml").Element("return_code").Value == "SUCCESS")
                {
                    WriteTxt(string.Format(@"商户订单号:{0},订单状态:{1},金额:{2},时间:{3},openid:{4}",
                        res.Element("xml").Element("out_trade_no").Value,
                        res.Element("xml").Element("result_code").Value,
                        res.Element("xml").Element("total_fee").Value,
                        res.Element("xml").Element("time_end").Value,
                        res.Element("xml").Element("openid").Value
                        ));
                    //保存订单信息，并返回微信应答
                    Response.Write(string.Format(@"<xml>
                               <return_code><![CDATA[SUCCESS]]></return_code>
                               <return_msg><![CDATA[OK]]></return_msg>
                            </xml>"));
                    //////////////////////////////////////////处理订单业务///////////////////////////////////
                    string orderid = Request.Cookies["orderid"].Value;
                    bool bol = oBLL.UpdateOrderState("2", orderid);
                    if (bol)
                    {
                        Response.Redirect("../AllOrder.aspx?sta=2");
                    }
                    else
                    {
                        Response.Write("<span style="color:#00CD00;font-size:20px">订单状态更新失败。</span><br/>");
                    }
                    /////////////////////////////////////////////////////////////////////////////////////////////
                }
                else
                {
                    WriteTxt(res.ToString());
                    Response.Write(string.Format(@"<xml>
                               <return_code><![CDATA[{0}]]></return_code>
                               <return_msg><![CDATA[{1}]]></return_msg>
                            </xml>", "FAIL", res.Element("xml").Element("return_msg").Value));
                }
            }
        }
    }
    /// <summary>
    /// 记录bug，以便调试
    /// </summary>
    /// <returns></returns>
    public bool WriteTxt(string str)
    {
        try
        {
            FileStream fs = new FileStream(Server.MapPath("success.txt"), FileMode.Append);
            StreamWriter sw = new StreamWriter(fs);
            //开始写入
            sw.WriteLine("");
            sw.WriteLine("--" + DateTime.Now.ToString("yyyy-MM-dd hh:mm:ss") + "--");
            sw.WriteLine("");
            sw.WriteLine(str);
            //清空缓冲区
            sw.Flush();
            //关闭流
            sw.Close();
            fs.Close();
        }
        catch (Exception)
        {
            return false;
        }
        return true;
    }
    
}