函数：根据Code值获取UnionId
/// <summary>
/// 调用微信接口 获取unionId
/// </summary>
/// <param name="code"></param>
/// <returns></returns>
public string GetWeChatUnionnId(string code)
{
#region 调用微信接口
//根据Code 获取refresh_token
var accessUrl = "https://api.weixin.qq.com/sns/oauth2/access_token?appid="
+ _WebWXAppId + "&secret=" + _WebWXAppSecrect + "&code=" + code
+ "&grant_type=authorization_code";
WebRequest accessRequest = WebRequest.Create(accessUrl);
StreamReader accessStream = new StreamReader(accessRequest.GetResponse().GetResponseStream());
var accessObjerct = Newtonsoft.Json.JsonConvert.DeserializeObject(accessStream.ReadToEnd());
var refreshToken = ((dynamic)accessObjerct).refresh_token;
var unionId = ((dynamic)accessObjerct).unionid;
if (refreshToken == null) return null;
//根据refresh_token重置access_token刷新时间 获取open_id
var accessRefreshUrl = "https://api.weixin.qq.com/sns/oauth2/refresh_token?appid="
+ _WebWXAppId + "&grant_type=refresh_token&refresh_token=" + (string)refreshToken;
WebRequest accessRefreshRequest = WebRequest.Create(accessRefreshUrl);
StreamReader accessRefershStream = new StreamReader(accessRefreshRequest.GetResponse().GetResponseStream());
var accessRefreshObjerct = Newtonsoft.Json.JsonConvert.DeserializeObject(accessRefershStream.ReadToEnd());
var accessToken = ((dynamic)accessRefreshObjerct).access_token;
if (accessToken == null || unionId == null) return null;
return unionId;
#endregion
}