启动一个任务的背景下，如果出现某些错误 [英] Restarting a task in the background if certain errors occur

问题描述

我使用的使用Mono.Mac（3.2.3）与服务器通信的一些休息的请求，并作为重试机制，我悄悄地试图给HTTP动作多次尝试，如果他们失败了，或超时。

I am using some REST requests using Mono.Mac (3.2.3) to communicate with a server, and as a retry mechanism I am quietly attempting to give the HTTP actions multiple tries if they fail, or time out.

我有以下;

var tries = 0;
while (tries <= ALLOWED_TRIES)
{
    try
    {
        postTask.Start();
        tries++;
        if (!postTask.Wait(Timeout))
        {
            throw new TimeoutException("Operation timed out");
        }
        break;
    } catch (Exception e) {
        if (tries > ALLOWED_TRIES)
        {
            throw new Exception("Failed to access Resource.", e);
        }
    }
}

当任务使用的参数像这样的父类的方法;

Where the task uses parameters of the parent method like so;

var postTask = new Task<HttpWebResponse>(() => {return someStuff(foo, bar);},
    Task.Factory.CancellationToken, 
    Task.Factory.CreationOptions);

这个问题似乎是，任务不希望与 postTask.Start（）后，它的第一个竣工（以及随后的故障）。是否有这样做的一个简单的方法，还是我用这种方式滥用的任务？是否有某种方式，该任务重置为初始状态，还是我最好使用一个工厂某种？

The problem seems to be that the task does not want to be run again with postTask.Start() after it's first completion (and subsequent failure). Is there a simple way of doing this, or am I misusing tasks in this way? Is there some sort of method that resets the task to its initial state, or am I better off using a factory of some sort?

推荐答案

您要真是滥用工作在这里，有几个原因：

You're indeed misusing the Task here, for a few reasons:

• 您不能运行相同的任务不止一次。当它这样做，它的完成。

• You cannot run the same task more than once. When it's done, it's done.

这是不建议手动构建一个工作对象，有 Task.Run 和 Task.Factory.Start 为

It is not recommended to construct a Task object manually, there's Task.Run and Task.Factory.Start for that.

您不应该使用 Task.Run / Task.Factory.Start 为其做任务IO绑定工作。它们主要用于CPU密集型的工作，因为他们借从线程池一个线程来执行任务操作。相反，用纯异步工作此为基础的API，它不需要奉献线程来完成。

You should not use Task.Run/Task.Factory.Start for a task which does IO-bound work. They are intended for CPU-bound work, as they "borrow" a thread from ThreadPool to execute the task action. Instead, use pure async Task-based APIs for this, which do not need a dedicate thread to complete.

例如，下面你可以从UI线程调用 GetResponseWithRetryAsync ，仍然保持UI响应：

For example, below you can call GetResponseWithRetryAsync from the UI thread and still keep the UI responsive:

async Task<HttpWebResponse> GetResponseWithRetryAsync(string url, int retries)
{
    if (retries < 0)
        throw new ArgumentOutOfRangeException();

    var request = WebRequest.Create(url);
    while (true)
    {
        try
        {
            var result = await request.GetResponseAsync();
            return (HttpWebResponse)result;
        }
        catch (Exception ex)
        {
            if (--retries == 0)
                throw; // rethrow last error
            // otherwise, log the error and retry
            Debug.Print("Retrying after error: " + ex.Message);
        }
    }
}

更​​多阅读：

More reading:

。Task.Factory.StartNew与新建任务（...）开始。

的 Task.Run VS Task.Factory.StartNew 。

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