在c#中调用外部API或者自己系统的API都是很简单的，这里提供使用HttpWebRequest的方式访问外部API，自定义了头部Access_key.
若外部系统没有此安全验证，可以去掉。

        public string OpenReadWithHttps(string URL, string strPostdata, string strEncoding)
        {
            Encoding encoding = Encoding.Default;
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(URL);
            request.Method = "post";
            request.Headers.Add("access_key", "your access_key");
            request.Accept = "application/json";
            request.ContentType = "application/json; charset=utf-8";
            byte[] buffer = Encoding.ASCII.GetBytes(strPostdata);
            request.ContentLength = buffer.Length;
            request.GetRequestStream().Write(buffer, 0, buffer.Length);
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            using (StreamReader reader = new StreamReader(response.GetResponseStream(), encoding))
            //System.Text.Encoding.GetEncoding(strEncoding)
            {
                return reader.ReadToEnd();
            }
        }