HTTPClient every time returns the same string

问题

Could some one make me clear why my code returns the same string every time?

public MainPage()
{
    this.InitializeComponent();

    DispatcherTimer timer = new DispatcherTimer();
    timer.Interval = TimeSpan.FromSeconds(5);
    timer.Tick += OnTimerTick;
    timer.Start();
}

private void OnTimerTick(object sender, object e)
{
    getData();
    HubText.Text = dumpstr;
}

private async void getData()
{
    // Create an HttpClient instance
    HttpClient client = new HttpClient();
    var uri = new Uri("http://192.168.4.160:8081/v");
    try
    {
        // Send a request asynchronously continue when complete
        HttpResponseMessage response = await client.GetAsync(uri);
        // Check that response was successful or throw exception
        response.EnsureSuccessStatusCode();
        // Read response asynchronously
        dumpstr = await response.Content.ReadAsStringAsync();
    }
    catch (Exception e)
    {
        //throw;
    }
}
string dumpstr;

So every 5 seconds I get the same string that I got in my first request. What am I doing wrong?

回答1:

It's because you're doing a GET to the same URL. According to the HTTP semantics, the value should be the same within a reasonable timeframe, so the OS is caching the response for you.

You can bypass the cache by any of these methods:

Using a POST request.
Adding a query string parameter that is different for each call.
Specifying (on the server) response headers that disable or limit the caching allowed.

回答2:

If you are using Windows.Web.Http.HttpClient, you can skip the local cache this way:

Windows.Web.Http.Filters.HttpBaseProtocolFilter filter =
    new Windows.Web.Http.Filters.HttpBaseProtocolFilter();
filter.CacheControl.ReadBehavior =
    Windows.Web.Http.Filters.HttpCacheReadBehavior.MostRecent;

HttpClient client = new HttpClient(filter);
Uri uri = new Uri("http://example.com");
HttpResponseMessage response = await client.GetAsync(uri);

response.EnsureSuccessStatusCode();
string str = await response.Content.ReadAsStringAsync();

You will never get the same response twice again :)

But if you have access to the server source code, the most elegant fix would be to disable the cache for the URI you are downloading, i.e., add the Cache-Control: no-cache header.

回答3:

I tried everything, this one worked for me. Just in case some is not able to make it work:

var uri = new Uri("http://192.168.4.160:8081/v?time=" + DateTime.Now);

来源：https://stackoverflow.com/questions/21643747/httpclient-every-time-returns-the-same-string

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