微信支付 申请退款 报(400) 错误的请求 ？

   var url = "https://api.mch.weixin.qq.com/secapi/pay/refund";

            string strA = "appid=" + appid + "&mch_id=" + mch_id + "&nonce_str=" + nonce_str + "&out_trade_no=" + out_trade_no

                + "&total_fee=" + total_fee + "&out_refund_no=" + out_refund_no + "&refund_fee=" + refund_fee;

            string strk = strA + "&key=" + _keyGroup; //key为商户平台设置的密钥key(假)

            string strMD5 = MD5(strk).ToUpper();//MD5签名 

            var formData = "<xml>";

            formData += "<appid>" + appid + "</appid>";//appid  

            formData += "<mch_id>" + mch_id + "</mch_id>";//商户号 

            formData += "<nonce_str>" + nonce_str + "</nonce_str>";//随机字符串，不长于32位。 

            formData += "<out_refund_no>" + out_refund_no + "</out_refund_no>";//商户退款单号

            formData += "<out_trade_no>" + out_trade_no + "</out_trade_no>";//商户订单号  --待   

            formData += "<refund_fee>" + refund_fee + "</refund_fee>";//退款金额

            formData += "<total_fee>" + total_fee + "</total_fee>";//支付金额单位为（分）

            formData += "<sign>" + strMD5 + "</sign>"; //签名

            formData += "</xml>";

            //请求数据

            var getdata = sendPost(url, formData);

/// <summary>

        /// wx统一下单请求数据

        /// </summary>

        /// <param name="URL">请求地址</param>

        /// <param name="urlArgs">参数</param>

        /// <returns></returns>

        private static string sendPost(string URL, string urlArgs)

        {

            //context.Request["args"]

            System.Net.WebClient wCient = new System.Net.WebClient();

            wCient.Headers.Add("Content-Type", "application/x-www-form-urlencoded");

            byte[] postData = System.Text.Encoding.UTF8.GetBytes("body=" + urlArgs);

            byte[] responseData = wCient.UploadData(URL, "POST", postData);

            string returnStr = System.Text.Encoding.UTF8.GetString(responseData);//返回接受的数据 

            return returnStr;

        }