using (var client = new HttpClient())             {

               //方法1：

                CacheControlHeaderValue cacheControl = new CacheControlHeaderValue();                 cacheControl.NoCache = true;                 cacheControl.NoStore = true;                 client.DefaultRequestHeaders.CacheControl = cacheControl;

               //方法2：

                //client.DefaultRequestHeaders.Add("Cache-Control", "no-cache");                 try                 {                     client.GetStringAsync(url);                     return true;                 }                 catch                 {                     return false;                 }             }