/// <summary>
    /// 验证消息的有效性
    /// </summary>
    /// <param name="context"></param>
    /// <returns>如果消息有效，返回true；否则返回false。</returns>
    private bool Validate(HttpContext context)
    {
        string username = RequestEx.TryGetQueryString("username");  //在接口配置的URL中加入了username参数，表示哪个微信公众号
        AccountInfo account = AccountInfoCollection.GetAccountInfo(username);
        if (account == null)
            return false;
        string token = account.Token;
        string signature = RequestEx.TryGetQueryString("signature");
        string timestamp = RequestEx.TryGetQueryString("timestamp");
        string nonce = RequestEx.TryGetQueryString("nonce");
        if (string.IsNullOrWhiteSpace(signature) || string.IsNullOrWhiteSpace(timestamp) || string.IsNullOrWhiteSpace(nonce))
            return false;
        return xrwang.weixin.PublicAccount.Utility.CheckSignature(signature, token, timestamp, nonce);
    }

RequestBaseMessage bm=helper.Message;
switch(bm.MsgType)
{
    case RequestMessageTypeEnum.text:  //文本消息
        HandleTextMessage((RequestTextMessage)bm);
        break;
    case RequestMessageTypeEnum.image: //图片消息
        HandleImageMessage((RequestImageMessage)bm);
        break;
    //处理其他消息
    case RequestMessageTypeEnum.event:    //事件
        RequestEventMessage ev=(RequestEventMessage)bm;
        switch(ev.Event)
        {
            case RequestEventTypeEnum.subscribe:    //订阅
                HandleSubscribeMessage((RequestSubscribeMessage)ev);
                break;
            case RequestEventTypeEnum.unsubscribe:    //取消订阅
                HandleUnsubscribeMessage((RequestUnsubscribeMessage)ev);
                break;
            //处理其他事件
        }
        break;
    default:
        break;
}

    /// <summary>
    /// 处理微信的POST请求
    /// </summary>
    /// <param name="context"></param>
    /// <returns>返回xml响应</returns>
    private string HandlePost(HttpContext context)
    {
        RequestMessageHelper helper = new RequestMessageHelper(context.Request);
        if (helper.Message != null)
        {
            ResponseBaseMessage responseMessage = HandleRequestMessage(helper.Message);
            return responseMessage.ToXml(helper.EncryptType);
        }
        else
            return string.Empty;
    }

    /// <summary>
    /// 处理请求消息，返回响应消息
    /// </summary>
    /// <returns>返回响应消息</returns>
    private ResponseBaseMessage HandleRequestMessage(RequestBaseMessage requestMessage)
    {
        ResponseTextMessage response = new ResponseTextMessage(requestMessage.FromUserName, requestMessage.ToUserName, 
            DateTime.Now, string.Format("自动回复，请求内容如下：\r\n{0}", requestMessage));
        return response;
    }