.NET HttpClient - 取消了CancellationToken而不取消请求

Question

我遇到了.NET HttpClient类(.NET 4.5.1,System.Net.Http v4.0.0.0)的问题.我正在调用HttpClient.GetAsync,传入一个CancellationToken(作为Nuget包的一部分,用于抽象webservices之间的调用).如果在进行调用之前已取消令牌,则请求将在不抛出异常的情况下通过.这种行为似乎不正确.

我的测试(不完整,未完全编写 - 无异常检查):

[TestMethod]
public async Task Should_Cancel_If_Cancellation_Token_Called()
{
    var endpoint = "nonexistent";
    var cancellationTokenSource = new CancellationTokenSource();

    var _mockHttpMessageHandler = new MockHttpMessageHandler();
    _mockHttpMessageHandler
        .When("*")
        .Respond(HttpStatusCode.OK);

    var _apiClient = new ApiClientService(new HttpClient(_mockHttpMessageHandler));
    cancellationTokenSource.Cancel();

    var result = await _apiClient.Get<string>(endpoint, null, cancellationTokenSource.Token);
}

我正在测试的方法:

public async Task<T> Get<T>(string endpoint, IEnumerable<KeyValuePair<string, string>> parameters = null, CancellationToken cancellationToken = default(CancellationToken))
{
    var builder = new UriBuilder(Properties.Settings.Default.MyEndpointHost + endpoint);
    builder.Query = buildQueryStringFromParameters(parameters);

    _httpClient.DefaultRequestHeaders.Accept.Clear();
    _httpClient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

    try
    {
        // After this, we really shouldn't continue.
        var request = await _httpClient.GetAsync(builder.Uri, cancellationToken);

        if (!request.IsSuccessStatusCode)
        {
            if (request.StatusCode >= HttpStatusCode.BadRequest && request.StatusCode < HttpStatusCode.InternalServerError)
            {
                throw new EndpointClientException("Service responded with an error message.", request.StatusCode, request.ReasonPhrase);
            }

            if (request.StatusCode >= HttpStatusCode.InternalServerError && (int)request.StatusCode < 600)
            {
                throw new EndpointServerException("An error occurred in the Service endpoint.", request.StatusCode, request.ReasonPhrase);
            }
        }

        var json = await request.Content.ReadAsStringAsync();
        return JsonConvert.DeserializeObject<T>(json);
    }
    catch (Exception ex)
    {
        throw;
    }
}

我知道我可以在调用之前检查取消令牌的状态,HttpClient.GetAsync如果已经请求取消则抛出.我知道我也可以注册一个委托来取消HttpClient请求.然而,似乎将令牌传递给HttpClient方法应该为我处理这个问题(或者,除此之外,重点是什么？)所以我想知道我是否遗漏了一些东西.我无权访问HttpClient源代码.

HttpClient.GetAsync当我传递它时,为什么不检查我的取消令牌并中止其进程？

Answer 1

HttpClient本身不检查取消标记，而是在调用其SendAsync方法时将其传递给消息处理程序。然后，它注册到从消息处理程序返回的任务的延续SendAsync，并且如果从消息处理程序返回的任务被取消，则将其自己的任务设置为已取消。

因此，您的场景中的问题在于您的实现MockHttpMessageHandler似乎没有检查取消令牌。

请注意，如果HttpClient通过其空构造函数调用，它会在内部使用HttpClientHandler它在取消令牌上注册一个委托，该委托将中止请求并取消任务。