C# Http post 怎么总是报错 {"errcode":47001,"errmsg":"?

代码如下：

..........

var httpWebRequest = (HttpWebRequest)WebRequest.Create(@"https://api.weixin.qq.com/tcb/databasequery?access_token="+accessToken);           

httpWebRequest.ContentType = "application/json;";

httpWebRequest.Method = “POST”;

using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))

            {

                string json = "{\"env\":\"*******************\",\"query\":\"db.collection(\"usersInfo\").limit(10).skip(0).get()\"}";

                streamWriter.Write(json);

                streamWriter.Flush();

                streamWriter.Close();

}

 var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();

            using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))

            {

                var result = streamReader.ReadToEnd();

                Console.WriteLine(result.ToString());

            }

..............

        }

求教！！！