WebClient/HttpClient缓存问题

我正在开发一个代码,每隔30秒轮询一次Bitstamp交换代码.这是我的代码:

public IObservable Stream(Uri way, WebClient wc)
    {
        Func, Task> Fun = async Observer =>
        {
            var res = await wc.DownloadStringTaskAsync(way);
            Observer.OnNext(value: res);
        };

        return Observable.Create(Fun);
    }

public IObservable GetDelay(int secs)
    {
        var exe = TimeSpan.FromSeconds(secs);
        return Observable.Empty("x").Delay(exe);
    }

Stream(new Uri("https://bitstamp.net/api/ticker"), new WebClient { }).Concat(GetDelay(30))
    .Repeat(5).Subscribe(res => Debug.WriteLine("got result: {0}", res));

问题是WebClient(并且HttpClient)也会在第一次调用后返回缓存的结果,可以通过相同的时间戳看到:

got result: {"high": "690.00", "last": "645.10", "timestamp": "1387715532" ... }
got result: {"high": "690.00", "last": "645.10", "timestamp": "1387715532" ... }
...

即使在关闭网络后,它们也会正常返回结果,所以很明显它们会将其缓存到某处.添加类似"？cache = random"的内容不起作用,因为Bitstamp上的ticker不允许使用请求参数.设置Headers[HttpRequestHeader.CacheControl] = "no-cache"了WebRequest也不起作用.

我该如何解决这种奇怪的缓存行为？