Caching issue with WebClient/HttpClient - caching

Question

I'm developing a code to poll a Bitstamp exchange ticker every 30 seconds. This is a code I have:
public IObservable<string> Stream(Uri way, WebClient wc)
{
Func<IObserver<string>, Task> Fun = async Observer =>
{
var res = await wc.DownloadStringTaskAsync(way);
Observer.OnNext(value: res);
};
return Observable.Create<string>(Fun);
}
public IObservable<string> GetDelay(int secs)
{
var exe = TimeSpan.FromSeconds(secs);
return Observable.Empty<string>("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));
The problem is that WebClient (and HttpClient, too) both return cached results after the first call, it can be seen by the same timestamp:
got result: {"high": "690.00", "last": "645.10", "timestamp": "1387715532" ... }
got result: {"high": "690.00", "last": "645.10", "timestamp": "1387715532" ... }
...
Even after turning the networks off they return the result normally so obviously they cache it somewhere. Adding something like "?cache=random" does not work because request parameters are not allowed for ticker on Bitstamp. Setting Headers[HttpRequestHeader.CacheControl] = "no-cache" for WebRequest does not work either.
How can I fix this weird caching behavior?

Answer 1

Solved by setting wc.Headers[HttpRequestHeader.IfModifiedSince] = DateTime.UtcNow.ToString(); before each subsequent call.

Answer 2

For Windows Phone Setting with Language=Chinese and "date+time" is not "24-hour clock" ,
the code wc.Headers[HttpRequestHeader.IfModifiedSince] = DateTime.UtcNow.ToString() causes exception throwing since DateTime.ToString() generates Chinese characters "下午" in the header.
The safer solution is to output the date time format as RFC1123 pattern:
req.Headers[HttpRequestHeader.IfModifiedSince] = DateTime.UtcNow.ToString("R");
This will ensure the date time is in format of "Sat, 05 Jul 2014 13:38:28 GMT", and there will no any Chinese characters within the HTTP headers.