Question

我试图运行下面的示例时遇到运行时异常。

Unhandled Exception: System.InvalidOperationException: The current SynchronizationContext may not be used as a TaskScheduler.
   at System.Threading.Tasks.SynchronizationContextTaskScheduler..ctor()
   at System.Threading.Tasks.TaskScheduler.FromCurrentSynchronizationContext()
   at TaskDemo.MyForm..ctor() in D:\myStudio\ASPNet\CSharp\CSharp4\MyApp\MyApp\Hello.cs:line 428
   at TaskDemo.SynchronizationContextTaskScheduler() in D:\myStudio\ASPNet\CSharp\CSharp4\MyApp\MyApp\Hello.cs:line 396
   at TaskDemo.Go() in D:\myStudio\ASPNet\CSharp\CSharp4\MyApp\CLRviaCSharp\Hello.cs:line 214
   at ComputeOps.Main() in D:\myStudio\ASPNet\CSharp\CSharp4\MyApp\CLRviaCSharp\Hello.cs:line 23

代码示例：

public class TaskSchedulerTest {

    public void Test() {
        SynchronizationContextTaskScheduler();
    }

    private void SynchronizationContextTaskScheduler() {
        var f = new MyForm();
        System.Windows.Forms.Application.Run();
    }

    private sealed class MyForm : System.Windows.Forms.Form {
        public MyForm() {
            Text = "Synchronization Context Task Scheduler Demo";
            Visible = true; Width = 400; Height = 100;
        }

        private readonly TaskScheduler m_syncContextTaskScheduler =
           TaskScheduler.FromCurrentSynchronizationContext();

        private CancellationTokenSource m_cts;

        protected override void OnMouseClick(System.Windows.Forms.MouseEventArgs e) {
            if (m_cts != null) {    // An operation is in flight, cancel it
                m_cts.Cancel();
                m_cts = null;
            } else {                // An operation is not in flight, start it
                Text = "Operation running";
                m_cts = new CancellationTokenSource();

                // This task uses the default task scheduler and executes on a thread pool thread
                var t = new Task<Int32>(() => Sum(m_cts.Token, 20000), m_cts.Token);
                t.Start();

                // These tasks use the synchronization context task scheduler and execute on the GUI thread
                t.ContinueWith(task => Text = "Result: " + task.Result,
                   CancellationToken.None, TaskContinuationOptions.OnlyOnRanToCompletion,
                   m_syncContextTaskScheduler);

                t.ContinueWith(task => Text = "Operation canceled",
                   CancellationToken.None, TaskContinuationOptions.OnlyOnCanceled,
                   m_syncContextTaskScheduler);

                t.ContinueWith(task => Text = "Operation faulted",
                   CancellationToken.None, TaskContinuationOptions.OnlyOnFaulted,
                   m_syncContextTaskScheduler);
            }
            base.OnMouseClick(e);
        }
    }
}

有什么想法吗？

Answer 1

将m_syncContextTaskScheduler的创建放入表单构造函数中。

public MyForm() {
    Text = "Synchronization Context Task Scheduler Demo";
    Visible = true; Width = 400; Height = 100;
    m_syncContextTaskScheduler = TaskScheduler.FromCurrentSynchronizationContext();
}

Answer 2

谢谢！它有效，背后的原因是什么？

因为构造函数很快初始化了具有初始化程序的 readonly 成员。如果需要，Form类构造函数安装同步提供程序，即名为WindowsFormsSynchronizationContext的类的实例。在调用基类构造函数之前，C＃编译器为readonly初始化器生成代码。将赋值移动到构造函数体中可确保在调用基础构造函数之后初始化它。

小心使用 readonly 成员初始值设定项，保持简单。

.NET中的TaskScheduler.FromCurrentSynchronizationContext（）

2 个答案: