TaskScheduler不在主线程上运行任务

Question

I write following code to try using TaskScheduler. Task content within the UpdateStatus function need to be run on the main thread. But I just get the output indicate that it's running on separate thread.

Any ideas about the reason? or alternative method of writing status from background thread in main thread context.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
  class Test
  {
    TaskScheduler scheduler;
    public void Run()
    {
      SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
      scheduler = TaskScheduler.FromCurrentSynchronizationContext();
      Console.WriteLine("Start on {0}", Thread.CurrentThread.ManagedThreadId);
      Task.Factory.StartNew(() =>
      {
        for (int i = 0; i < 10; i++)
        {
          Console.WriteLine("running on {0}", Thread.CurrentThread.ManagedThreadId);
          UpdateStatus(string.Format("Message {0}", i));
          Thread.Sleep(1000);
        }
      }).ContinueWith(_ =>
      {
        Console.WriteLine("complate");
        Console.WriteLine("running on {0}", Thread.CurrentThread.ManagedThreadId);
      }, new CancellationToken(), TaskContinuationOptions.None, scheduler).Wait();
    }

    private void UpdateStatus(string message)
    {
      Task.Factory.StartNew(() =>
      {
        Console.WriteLine("updating status on {0}", Thread.CurrentThread.ManagedThreadId);
        Console.WriteLine(message);
      }, new CancellationToken(), TaskCreationOptions.None, scheduler);
    }
  }

  class Program
  {
    public static void Main(string[] args)
    {
      var test = new Test();
      test.Run();
    }
  }
}

Answer 1

The Default SynchronizationContext (which you are using), doesn't get you to the main thread. See Why SynchronizationContext does not work properly? .

As svick mentions, in your case (a Console application), there is usually no need for the concept of a main thread.

Answer 2

You didn't explain what exactly are you trying to do, so it's hard to advise you on the best course of action.

Your current code doesn't work, because the SynchronizationContext you're creating doesn't actually do any synchronization. (And your main thread is also blocked on the Wait() call, so nothing else can run on it.)

You could use synchronization context that synchronizes properly to the main thread (like the one from Nito AsyncEx ), but I'm not sure that's the best way in your case.

I think a better approach would be to use the producer/consumer pattern using BlockingCollection . Your Task s would add items to it and your main thread would take them from there and send them to the pipeline using WriteObject() .