C# HttpClient禁止缓存

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;
}
}