c# – 我如何允许任务异常传播回UI线程？

Ok Joe …如所承诺的,下面是如何通过自定义TaskScheduler子类来一般性地解决这个问题.我已经测试了这个实现,它的作用就像一个魅力.不要忘记,如果你想看到Application.ThreadException实际上启动,你不能附加调试器！

自定义TaskScheduler

这个自定义TaskScheduler实现被绑定到一个特定的SynchronizationContext在“诞生”,并将采取每个传入的任务,它需要执行,链接一个继续,只会触发逻辑任务故障,当它触发时,它返回到SynchronizationContext,它将从任务中抛出异常.

public sealed class SynchronizationContextFaultPropagatingTaskScheduler : TaskScheduler
{
    #region Fields

    private SynchronizationContext synchronizationContext;
    private ConcurrentQueue<Task> taskQueue = new ConcurrentQueue<Task>();

    #endregion

    #region Constructors

    public SynchronizationContextFaultPropagatingTaskScheduler() : this(SynchronizationContext.Current)
    {
    }

    public SynchronizationContextFaultPropagatingTaskScheduler(SynchronizationContext synchronizationContext)
    {
        this.synchronizationContext = synchronizationContext;
    }

    #endregion

    #region Base class overrides

    protected override void QueueTask(Task task)
    {
        // Add a continuation to the task that will only execute if faulted and then post the exception back to the synchronization context
        task.ContinueWith(antecedent =>
            {
                this.synchronizationContext.Post(sendState =>
                {
                    throw (Exception)sendState;
                },antecedent.Exception);
            },TaskContinuationOptions.OnlyOnFaulted | TaskContinuationOptions.ExecuteSynchronously);

        // Enqueue this task
        this.taskQueue.Enqueue(task);

        // Make sure we're processing all queued tasks
        this.EnsureTasksAreBeingExecuted();
    }

    protected override bool TryExecuteTaskInline(Task task,bool taskWasPreviouslyQueued)
    {
        // Excercise for the reader
        return false;
    }

    protected override IEnumerable<Task> GetScheduledTasks()
    {
        return this.taskQueue.ToArray();
    }

    #endregion

    #region Helper methods

    private void EnsureTasksAreBeingExecuted()
    {
        // Check if there's actually any tasks left at this point as it may have already been picked up by a previously executing thread pool thread (avoids queueing something up to the thread pool that will do nothing)
        if(this.taskQueue.Count > 0)
        {
            ThreadPool.UnsafeQueueUserWorkItem(_ =>
            {
                Task nextTask;

                // This thread pool thread will be used to drain the queue for as long as there are tasks in it
                while(this.taskQueue.TryDequeue(out nextTask))
                {
                    base.TryExecuteTask(nextTask);
                }
            },null);
        }
    }

    #endregion
}

有关此实施的注释/免责声明：

>如果使用无参数的构造函数,它将会拾取当前的SynchronizationContext …,所以如果你只是在一个WinForms线程(主窗体构造函数,无论什么)构造它,它将自动工作.奖金,我也有一个构造函数,你可以在其中明确地传入你从别的地方得到的SynchronizationContext.
>我没有提供TryExecuteTaskInline的实现,所以这个实现将只是排队要处理的任务.我把这作为读者的练习.这不难,只是…没有必要展示你要求的功能.
>我使用简单/原始的方法来调度/执行利用ThreadPool的任务.肯定有更丰富的实现,但是这个实现的重点只是将异常封装到“应用程序”线程

好的,现在你有几个选项来使用这个TaskScheduler：

预配置TaskFactory实例

此方法允许您一次设置TaskFactory,然后从该工厂实例开始的任何任务将使用自定义TaskScheduler.这基本上看起来像这样：

在应用程序启动时

private static readonly TaskFactory MyTaskFactory = new TaskFactory(new SynchronizationContextFaultPropagatingTaskScheduler());

整个代码

MyTaskFactory.StartNew(_ =>
{
    // ... task impl here ...
});

显式TaskScheduler每次呼叫

另一种方法是创建自定义TaskScheduler的实例,然后在每次启动任务时将其传递到默认的TaskFactory上的StartNew中.

在应用程序启动时

private static readonly SynchronizationContextFaultPropagatingTaskScheduler MyFaultPropagatingTaskScheduler = new SynchronizationContextFaultPropagatingTaskScheduler();

整个代码

Task.Factory.StartNew(_ =>
{
    // ... task impl here ...
},CancellationToken.None // your specific cancellationtoken here (if any)
TaskCreationOptions.None,// your proper options here
MyFaultPropagatingTaskScheduler);