.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);
 }
}