C＃Task.WhenAll在所有任务完成之前返回？

Question

C＃全新。

我正在使用TPL测试代理服务器列表的有效性。目前我只使用localhost + Fiddler进行测试。 Task.WhenAll在我的任务列表实际完成之前完成。我在SO上发现a similar question但不幸的是，接受的答案是不适用于我的案例的，因为我没有像那个问题的OP那样调用异步无效方法。

致电代码：

   public class Proxy
    {
        public string IP { get; set; }
        public int Port { get; set; }
        public bool IsValid { get; set; }

        public Proxy(string ip, int port)
        {
            //TODO Validate input.
            this.IP = ip;
            this.Port = port;
        }

        public async Task<bool> TestValidityAsync(ProxyTest test)
        {
            try
            {
                var req = HttpWebRequest.Create(test.URL);
                req.Proxy = new WebProxy(this.ToString());
                var resp = await req.GetResponseAsync();
                using (var sr = new StreamReader(resp.GetResponseStream()))
                {
                    var respBody = await sr.ReadToEndAsync();
                    if (respBody.Contains(test.Validation))
                    {
                        return true;
                    }
                    else
                    {
                        return false;
                    }
                }
            }
            catch (Exception)
            {
                return false;
            }
        }

        public override string ToString()
        {
            return this.IP + ":" + this.Port;
        }

    }

public class ProxyTest
{
    public string URL { get; set; }
    public string Validation { get; set; }
    public string Invalidation { get; set; }

    public ProxyTest(string url, string validation)
    {
        this.URL = url;
        this.Validation = validation;
    }

    public ProxyTest(string url, string validation, string invalidation)
    {
        this.URL = url;
        this.Validation = validation;
        this.Invalidation = invalidation;
    }
}

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