HTTPClient每次返回相同的字符串

Question

有人可以让我清楚为什么我的代码每次返回相同的字符串？

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;

所以每隔5秒我就得到第一个请求中的字符串.我究竟做错了什么？

Answer 1

如果您正在使用Windows.Web.Http.HttpClient,则可以通过以下方式跳过本地缓存:

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();

你永远不会再两次得到相同的回应:)

但是,如果您可以访问服务器源代码,那么最优雅的修复方法是禁用正在下载的URI的缓存,即添加Cache-Control: no-cache标头.

Answer 2

这是因为你正在对同一个URL进行GET.根据HTTP语义,值应该在合理的时间范围内相同,因此操作系统会为您缓存响应.

您可以通过以下任何方法绕过缓存:

• 使用POST请求.
• 添加每个调用不同的查询字符串参数.
• 指定(在服务器上)禁用或限制允许的高速缓存的响应标头.