C＃代理验证错误

Question

我想使用Web代理执行获取请求，但一直收到代码407代理身份验证错误。

async static void GetRequestProxy()
    {
        HttpClientHandler handler = new HttpClientHandler();
        WebProxy proxy = new WebProxy(new Uri("http://ip:host"));
        proxy.Credentials = new NetworkCredential("user", "password");
        WebRequest.DefaultWebProxy = proxy;
        handler.Proxy = proxy;
        using (var client = new HttpClient(handler))
        {
            string geturl = "http://google.com";
            client.DefaultRequestHeaders.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36");
            using (HttpResponseMessage response = await client.GetAsync(geturl))
            {
                Console.WriteLine(response.StatusCode);
            }
        }
    }