Content-Type httpClient使用c＃获取响应

Question

我需要使用以下标题从url获取响应：

HttpClient httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Add("Content-Type", "application/json");
httpClient.DefaultRequestHeaders.Add("X-apiKey", App.API_Key);
string responseAsString = await httpClient.GetStringAsync("http://......./");

它在此行停止运行，没有任何responseAsString值：

 httpClient.DefaultRequestHeaders.Add("Content-Type", "application/json");