参考微信开发文档：

https://developers.weixin.qq.com/miniprogram/dev/api/custommsg/callback_help.html

代码可用

/// <summary>
/// 接口认证
/// </summary>
/// <param name="echostr"></param>
/// <param name="signature"></param>
/// <param name="timestamp"></param>
/// <param name="nonce"></param>
/// <returns></returns>
[HttpGet]
public async Task<HttpResponseMessage> wx(string echostr, string signature, string timestamp, string nonce)
{
    string token = "token";
    if (!CheckSignature(token, signature, timestamp, nonce))
        echostr = "验证不正确";
    HttpResponseMessage responseMessage = new HttpResponseMessage { Content = new StringContent(echostr, Encoding.GetEncoding("UTF-8"), "text/plain") };
 
    return responseMessage;
}
/// <summary>
/// 接收客服消息
/// </summary>
/// <returns></returns>
[HttpPost]
public async Task<HttpResponseMessage> wx()
{
    LogHelper log = new LogHelper();
    var content = await Request.Content.ReadAsStringAsync();
    log.LogError(content);
    HttpResponseMessage responseMessage = new HttpResponseMessage();
    return responseMessage;
}
/// <summary>
/// 验证微信签名
/// </summary>
private bool CheckSignature(string token, string signature, string timestamp, string nonce)
{
    string[] ArrTmp = { token, timestamp, nonce };
 
    Array.Sort(ArrTmp);
    string tmpStr = string.Join("", ArrTmp);
    var data = SHA1.Create().ComputeHash(Encoding.UTF8.GetBytes(tmpStr));
    var sb = new StringBuilder();
    foreach (var t in data)
    {
        sb.Append(t.ToString("X2"));
    }
    tmpStr = sb.ToString();
    tmpStr = tmpStr.ToLower();
 
    if (tmpStr == signature)
    {
        return true;
    }
    else
    {
        return false;
    }
}

 推荐代码：使用 Senparc.Weixin SDK

该版本是用一般处理程序做的，当初在微信配置中填的是此地址，没有修改成WebApi。大家可以参考做成WebApi的

引用：

using Senparc.Weixin;
using Senparc.Weixin.MP;
using Senparc.Weixin.MP.Entities.Request;

public class wxapi : IHttpHandler
    {
        //public static readonly string Token = Config.SenparcWeixinSetting.Token;//与微信公众账号后台的Token设置保持一致，区分大小写。
        //public static readonly string EncodingAESKey = Config.SenparcWeixinSetting.EncodingAESKey;//与微信公众账号后台的EncodingAESKey设置保持一致，区分大小写。
        //public static readonly string AppId = Config.SenparcWeixinSetting.WeixinAppId;//与微信公众账号后台的AppId设置保持一致，区分大小写。
        public static string Token = CachedConfigContext.Current.SettingConfig.token;
        public static string EncodingAESKey = CachedConfigContext.Current.SettingConfig.EncodingAESKey;
        public static string AppId = CachedConfigContext.Current.SettingConfig.appId;

        readonly Func<string> _getRandomFileName = () => DateTime.Now.ToString("yyyyMMdd-HHmmss") + Guid.NewGuid().ToString("n").Substring(0, 6);

        public void ProcessRequest(HttpContext context)
        {
            string postString = string.Empty;

            PostModel postModel = new PostModel();
            postModel.Signature = HttpContext.Current.Request.QueryString["signature"];
            postModel.Timestamp = HttpContext.Current.Request.QueryString["timestamp"];
            postModel.Nonce = HttpContext.Current.Request.QueryString["nonce"];
            postModel.Msg_Signature = HttpContext.Current.Request.QueryString["msg_signature"];
            var openid = HttpContext.Current.Request.QueryString["openid"];
            var encrypt_type = HttpContext.Current.Request.QueryString["encrypt_type"];

            if (HttpContext.Current.Request.HttpMethod.ToUpper() == "POST")
            {
                //using (Stream stream = HttpContext.Current.Request.InputStream)
                //{
                //    Byte[] postBytes = new Byte[stream.Length];
                //    stream.Read(postBytes, 0, (Int32)stream.Length);
                //    postString = Encoding.UTF8.GetString(postBytes);
                //}

                //if (!string.IsNullOrEmpty(postString))
                //{
                //    //Execute(postString);
                //}
                if (!CheckSignature.Check(postModel.Signature, postModel.Timestamp, postModel.Nonce, Token))
                {
                    HttpContext.Current.Response.Write("error");
                    HttpContext.Current.Response.End();
                }
                #region 打包 PostModel 信息

                postModel.Token = Token;//根据自己后台的设置保持一致
                postModel.EncodingAESKey = EncodingAESKey;//根据自己后台的设置保持一致
                postModel.AppId = AppId;//根据自己后台的设置保持一致

                #endregion

                //v4.2.2之后的版本，可以设置每个人上下文消息储存的最大数量，防止内存占用过多，如果该参数小于等于0，则不限制
                var maxRecordCount = 10;

                //自定义MessageHandler，对微信请求的详细判断操作都在这里面。
                var messageHandler = new CustomMessageHandler(HttpContext.Current.Request.InputStream, postModel, maxRecordCount);

                #region 设置消息去重

                /* 如果需要添加消息去重功能，只需打开OmitRepeatedMessage功能，SDK会自动处理。
                 * 收到重复消息通常是因为微信服务器没有及时收到响应，会持续发送2-5条不等的相同内容的RequestMessage*/
                messageHandler.OmitRepeatedMessage = true;//默认已经开启，此处仅作为演示，也可以设置为false在本次请求中停用此功能

                #endregion

                try
                {

                    #region 记录 Request 日志

                    var logPath = HttpContext.Current.Server.MapPath(string.Format("~/logs/MP/{0}/", DateTime.Now.ToString("yyyy-MM-dd")));
                    if (!Directory.Exists(logPath))
                    {
                        Directory.CreateDirectory(logPath);
                    }

                    //测试时可开启此记录，帮助跟踪数据，使用前请确保logs文件夹存在，且有读写权限。
                    messageHandler.RequestDocument.Save(Path.Combine(logPath, string.Format("{0}_Request_{1}_{2}.txt", _getRandomFileName(),
                        messageHandler.RequestMessage.FromUserName,
                        messageHandler.RequestMessage.MsgType)));
                    if (messageHandler.UsingEcryptMessage)
                    {
                        messageHandler.EcryptRequestDocument.Save(Path.Combine(logPath, string.Format("{0}_Request_Ecrypt_{1}_{2}.txt", _getRandomFileName(),
                            messageHandler.RequestMessage.FromUserName,
                            messageHandler.RequestMessage.MsgType)));
                    }

                    #endregion

                    //执行微信处理过程
                    messageHandler.Execute();

                    #region 记录 Response 日志

                    //测试时可开启，帮助跟踪数据

                    //if (messageHandler.ResponseDocument == null)
                    //{
                    //    throw new Exception(messageHandler.RequestDocument.ToString());
                    //}
                    if (messageHandler.ResponseDocument != null)
                    {
                        messageHandler.ResponseDocument.Save(Path.Combine(logPath, string.Format("{0}_Response_{1}_{2}.txt", _getRandomFileName(),
                            messageHandler.ResponseMessage.ToUserName,
                            messageHandler.ResponseMessage.MsgType)));
                    }

                    if (messageHandler.UsingEcryptMessage && messageHandler.FinalResponseDocument != null)
                    {
                        //记录加密后的响应信息
                        messageHandler.FinalResponseDocument.Save(Path.Combine(logPath, string.Format("{0}_Response_Final_{1}_{2}.txt", _getRandomFileName(),
                            messageHandler.ResponseMessage.ToUserName,
                            messageHandler.ResponseMessage.MsgType)));
                    }

                    #endregion

                    //return Content(messageHandler.ResponseDocument.ToString());//v0.7-
                    //return new WeixinResult(messageHandler);//v0.8+
                    var res = new Senparc.Weixin.MP.MvcExtension.FixWeixinBugWeixinResult(messageHandler);//为了解决官方微信5.0软件换行bug暂时添加的方法，平时用下面一个方法即可
                    HttpContext.Current.Response.Write(res.Content);
                    HttpContext.Current.Response.End();
                }
                catch (Exception ex)
                {
                    #region 异常处理
                    WeixinTrace.Log("MessageHandler错误：{0}", ex.Message);

                    using (TextWriter tw = new StreamWriter(HttpContext.Current.Server.MapPath("~/logs/Error_" + _getRandomFileName() + ".txt")))
                    {
                        tw.WriteLine("ExecptionMessage:" + ex.Message);
                        tw.WriteLine(ex.Source);
                        tw.WriteLine(ex.StackTrace);
                        //tw.WriteLine("InnerExecptionMessage:" + ex.InnerException.Message);

                        if (messageHandler.ResponseDocument != null)
                        {
                            tw.WriteLine(messageHandler.ResponseDocument.ToString());
                        }

                        if (ex.InnerException != null)
                        {
                            tw.WriteLine("========= InnerException =========");
                            tw.WriteLine(ex.InnerException.Message);
                            tw.WriteLine(ex.InnerException.Source);
                            tw.WriteLine(ex.InnerException.StackTrace);
                        }

                        tw.Flush();
                        tw.Close();
                    }
                    HttpContext.Current.Response.Write("");
                    HttpContext.Current.Response.End();
                    #endregion
                }

            }
            else
            {
                if (CheckSignature.Check(postModel.Signature, postModel.Timestamp, postModel.Nonce, Token))
                {
                    string echostr = HttpContext.Current.Request.QueryString["echoStr"];
                    //返回随机字符串则表示验证通过
                    HttpContext.Current.Response.Write(echostr);
                    HttpContext.Current.Response.End();
                }
            }
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }