.net微信开发 如何获取AccessToken

public static Access_token GetAccessToken(){  string formatString = String.Format("https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid={0}&secret={1}", AppId, AppSecret);  Access_token res = new Access_token();  HttpWebRequest request = (HttpWebRequest)WebRequest.Create(formatString);  request.Method = "GET";  request.ContentType = "text/html;charset=UTF-8";  HttpWebResponse response = (HttpWebResponse)request.GetResponse();  Stream myResponseStream = response.GetResponseStream();  StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding("utf-8"));  string retString = myStreamReader.ReadToEnd();  myStreamReader.Close();  myResponseStream.Close();  if (retString.IndexOf("7200") > 0)  {    Access_token token = new Access_token();    token = JsonHelper.ParseFromJson<Access_token>(retString);    res.access_token = token.access_token;    res.expires_in = token.expires_in;  }  return res;}

public class Access_token{  public Access_token()  {    //     //TODO:用于验证Access_token是否过期实体    //   }  string _access_token;  string _expires_in;  /// <summary>   /// 获取到的凭证    /// </summary>   public string access_token  {    get { return _access_token; }    set { _access_token = value; }  }  /// <summary>   /// 凭证有效时间，单位：秒   /// </summary>   public string expires_in  {    get { return _expires_in; }    set { _expires_in = value; }  }}

/// <summary> /// 将JSON对象转换为Model/// </summary> /// <typeparam name="T"></typeparam> /// <param name="szJson"></param> /// <returns></returns> public static T ParseFromJson<T>(string szJson){  T obj = Activator.CreateInstance<T>();  using (MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(szJson)))  {    DataContractJsonSerializer serializer = new DataContractJsonSerializer(obj.GetType());    return (T)serializer.ReadObject(ms);  }}