HttpClient的 - 一个任务被取消？ [英] HttpClient - A task was cancelled?

问题描述

有一个或两个任务时，却抛出任务被取消的时候，我们有一个以上的任务中列出的错误它工作正常。

 清单<任务> allTask​​s =新的List<任务>（）; 
 allTask​​s.Add（....）; 
 allTask​​s.Add（....）; 
 Task.WaitAll（allTask​​s.ToArray（），configuration.CancellationToken）; 
 
 
私人静态任务< T> HttpClientSendAsync< T>（字符串URL，对象数据，列举HTTPMethod方法，字符串的contentType，令牌的CancellationToken）
 {
 HttpRequestMessage httpRequestMessage =新HttpRequestMessage（方法，URL）; 
的HttpClient HttpClient的=新的HttpClient（）; 
 httpClient.Timeout =新的时间跨度（Constants.TimeOut）; 
 
如果（数据！= NULL）
 {
字节[]的字节数组= Encoding.ASCII.GetBytes（Helper.ToJSON（数据））; 
的MemoryStream MemoryStream的=新的MemoryStream（字节阵列）; 
 httpRequestMessage.Content =新的StringContent（新的StreamReader（的MemoryStream）.ReadToEnd（），Encoding.UTF8，则contentType）; 
} 
 
返回httpClient.SendAsync（httpRequestMessage）.ContinueWith（任务=> 
 {
 VAR响应= task.Result; 
返回响应。 。Content.ReadAsStringAsync（）ContinueWith（stringTask => 
 {
 VAR JSON = stringTask.Result; 
返回Helper.FromJSON< T>（JSON）; 
}）; 
}）展开（）; 
} 
  

解决方案

有2可能的原因，一个 TaskCanceledException 将被抛出：

1. 一种叫取消（ ）在 CancellationTokenSource 与完成任务之前取消令牌关联。


2. 请求超时，即没有你 HttpClient.Timeout 指定的时间跨度内完成。

我的猜测是，这是一个超时。 （如果这是明确的取消，你可能会想通了这一点。）你可以更加确定通过检查异常：

 试
 {
 VAR响应= task.Result; 
} 
赶上（TaskCanceledException前）
 {
 //这里检查ex.CancellationToken.IsCancellationRequested。 
 //如果是假，这是相当安全的假设这是一个超时。 
} 
  

It works fine when have one or two tasks however throws an error "A task was cancelled" when we have more than one task listed.

List<Task> allTasks = new List<Task>();
allTasks.Add(....);
allTasks.Add(....);
Task.WaitAll(allTasks.ToArray(), configuration.CancellationToken);


private static Task<T> HttpClientSendAsync<T>(string url, object data, HttpMethod method, string contentType, CancellationToken token)
{
    HttpRequestMessage httpRequestMessage = new HttpRequestMessage(method, url);
    HttpClient httpClient = new HttpClient();
    httpClient.Timeout = new TimeSpan(Constants.TimeOut);

    if (data != null)
    {
        byte[] byteArray = Encoding.ASCII.GetBytes(Helper.ToJSON(data));
        MemoryStream memoryStream = new MemoryStream(byteArray);
        httpRequestMessage.Content = new StringContent(new StreamReader(memoryStream).ReadToEnd(), Encoding.UTF8, contentType);
    }

    return httpClient.SendAsync(httpRequestMessage).ContinueWith(task =>
    {
        var response = task.Result;
        return response.Content.ReadAsStringAsync().ContinueWith(stringTask =>
        {
            var json = stringTask.Result;
            return Helper.FromJSON<T>(json);
        });
    }).Unwrap();
}

解决方案

There's 2 likely reasons that a TaskCanceledException would be thrown:

1. Something called Cancel() on the CancellationTokenSource associated with the cancellation token before the task completed.
2. The request timed out, i.e. didn't complete within the timespan you specified on HttpClient.Timeout.

My guess is it was a timeout. (If it was an explicit cancellation, you probably would have figured that out.) You can be more certain by inspecting the exception:

try
{
    var response = task.Result;
}
catch (TaskCanceledException ex)
{
    // Check ex.CancellationToken.IsCancellationRequested here.
    // If false, it's pretty safe to assume it was a timeout.
}

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