在使用获取 access_token方法的时候报这个异常： An unhandled exception of type 'System.AggregateException' occurred in mscorlib.dll，
怎么解？请求大神帮吗解决

我用的是 c# 的调用，以下是 C# 的 API 文档中的调用代码，测试的时候会在红色字体的地方报这个异常

String authHost = "https://aip.baidubce.com/oauth/2.0/token";
HttpClient client = new HttpClient();
List> paraList = new List>();
paraList.Add(new KeyValuePair("grant_type", "client_credentials"));
paraList.Add(new KeyValuePair("client_id", clientId));
paraList.Add(new KeyValuePair("client_secret", clientSecret));

HttpResponseMessage response = client.PostAsync(authHost, new FormUrlEncodedContent(paraList)).Result;
result = response.Content.ReadAsStringAsync().Result;
Console.WriteLine(result);
return result;