任务继续已安排到非线程池线程.为什么?

在控制台应用程序中，我确实创建了自己的线程来实现工作队列。此外，我已经为这个唯一的线程实现了我自己的SynchronizationContext。

当我等待来自主线程的任务时，突然在我的工作线程上安排了继续（我例程的剩余部分），这是错误的，因为我不希望我的线程被用作随机任务的线程池线程。

只有在使用Mono运行代码时，我才会遇到这种行为。

下面是一个代码，它再现了mono上的问题（在macosx和linux系统上测试）：

using System;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
class Program
{
    static void Main( string[] args )
    {
        Foo();
        Console.ReadLine();
    }

    async static void Foo()
    {
        Console.WriteLine( "{0}: current thread ID={1}; scheduler={2}; context={3};",
            "   Main BEFORE awaiting",
            Thread.CurrentThread.ManagedThreadId, 
            TaskScheduler.Current.Id,
            SynchronizationContext.Current != null );
            // MONO Output: Main BEFORE awaiting: current thread ID=1; scheduler=1; context=False;
        WorkQueue queue = new WorkQueue();
        // !!! 
        // I do expect that current context which is null will be captured for continuation.
        // !!!
        await queue.Enqueue();
        // !!!
        // As we can see our custom context was captured to continue with this part of code.
        // 
        Console.WriteLine( "{0}: current thread ID={1}; scheduler={2}; context={3};",
            "   Main AFTER awaiting",
            Thread.CurrentThread.ManagedThreadId,
            TaskScheduler.Current.Id,
            SynchronizationContext.Current != null );
        // MONO Output: Main AFTER awaiting: current thread ID=4; scheduler=1; context=True;
    }
}
// Custom context which does nothing but enqueues fake tasks to the queue.
//
class WorkQueueSyncContext : SynchronizationContext
{
    readonly WorkQueue queue;
    public WorkQueueSyncContext( WorkQueue queue )
    {
        this.queue = queue;
    }
    public override void Post( SendOrPostCallback d, object state )
    {
    }
    public override void Send( SendOrPostCallback d, object state )
    {
        queue.Enqueue().Wait();
    }
}
// The queue
//
class WorkQueue
{
    readonly Thread thread;
    class WorkQueueItem
    {
        public TaskCompletionSource<object> Completion
        {
            get;
            set;
        }
    }
    BlockingCollection<WorkQueueItem> queue = new BlockingCollection<WorkQueueItem>();

    public WorkQueue()
    {
        thread = new Thread( new ThreadStart( Run ) );
        thread.Start();
    }
    private void Run()
    {
        // Set ower own SynchronizationContext.
        //
        SynchronizationContext.SetSynchronizationContext( new WorkQueueSyncContext( this ) );
        Console.WriteLine( "{0}: current thread ID={1}; scheduler={2}; context={3};",
            "   WorkQueue START",
            Thread.CurrentThread.ManagedThreadId,
            TaskScheduler.Current.Id,
            SynchronizationContext.Current != null );
        // MONO Output: current thread ID=4; scheduler=1; context=True;
        // Working loop.
        //
        while ( true )
        {
            WorkQueueItem item = queue.Take();
            Console.WriteLine( "{0}: current thread ID={1}; scheduler={2}; context={3};",
                "   WorkQueue DOING TASK",
                Thread.CurrentThread.ManagedThreadId,
                TaskScheduler.Current.Id,
                SynchronizationContext.Current != null );
            // MONO Output: current thread ID=4; scheduler=1; context=True;
            // Completed the task :)
            //
            item.Completion.SetResult( true );
        }
    }
    public Task<object> Enqueue()
    {
        TaskCompletionSource<object> completion = new TaskCompletionSource<object>();
        queue.Add( new WorkQueueItem() { Completion = completion } );
        return completion.Task;
    }
}

所以，这里是MONO输出：

   Main BEFORE awaiting: current thread ID=1; scheduler=1; context=False;
   WorkQueue START: current thread ID=3; scheduler=1; context=True;
   WorkQueue DOING TASK: current thread ID=3; scheduler=1; context=True;
   Main AFTER awaiting: current thread ID=3; scheduler=1; context=True;

这是Windows的输出：

   Main BEFORE awaiting: current thread ID=10; scheduler=1; context=False;
   WorkQueue START: current thread ID=11; scheduler=1; context=True;
   WorkQueue DOING TASK: current thread ID=11; scheduler=1; context=True;
   Main AFTER awaiting: current thread ID=6; scheduler=1; context=False;

请注意（最后一行）上下文捕获的不同之处。

编辑：

Mono 3.4.0不可复制，因此在旧版本（至少3.2.6）中似乎是一个错误；