主线程的SynchronizationContext.Current如何在Windows窗体应用程序中变为空？

Question

我的应用程序有问题:在某些时候,SynchronizationContext.Current对主线程变为null.我无法在一个孤立的项目中重现同样的问题.我的真实项目很复杂; 它混合使用Windows窗体和WPF并调用WCF Web服务.据我所知,这些都是可以与SynchronizationContext交互的系统.

这是我孤立项目的代码.我真正的应用程序做了类似的事情.但是,在我的实际应用程序中,执行继续任务时,主线程上的SynchronizationContext.Current为null.

private void button2_Click(object sender, EventArgs e)
{
    if (SynchronizationContext.Current == null)
    {
        Debug.Fail("SynchronizationContext.Current is null");
    }

    Task.Factory.StartNew(() =>
    {
        CallWCFWebServiceThatThrowsAnException();
    })
    .ContinueWith((t) =>
    {

        //update the UI
        UpdateGUI(t.Exception);

        if (SynchronizationContext.Current == null)
        {
            Debug.Fail("SynchronizationContext.Current is null");
        }

    }, CancellationToken.None, 
       TaskContinuationOptions.OnlyOnFaulted,
       TaskScheduler.FromCurrentSynchronizationContext());
}

什么可能导致主线程的SynchronizationContext.Current变为null？

编辑:

@Hans要求堆栈跟踪.这里是:

   at MyApp.Framework.UI.Commands.AsyncCommand.HandleTaskError(Task task) in d:\sources\s2\Framework\Sources\UI\Commands\AsyncCommand.cs:line 157
   at System.Threading.Tasks.Task.c__DisplayClassb.b__a(Object obj)
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.Tasks.Task.ExecutionContextCallback(Object obj)
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean bPreventDoubleExecution)
   at System.Threading.Tasks.SynchronizationContextTaskScheduler.PostCallback(Object obj)
   at System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
   at System.RuntimeMethodHandle.InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeType typeOwner)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks)
   at System.Delegate.DynamicInvokeImpl(Object[] args)
   at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)
   at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
   at System.Threading.ExecutionContext.runTryCode(Object userData)
   at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)
   at System.Windows.Forms.Control.InvokeMarshaledCallbacks()
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at MyApp.Framework.SharedUI.ApplicationBase.InternalStart() in d:\sources\s2\Framework\Sources\UI\SharedUI\ApplicationBase.cs:line 190
   at MyApp.Framework.SharedUI.ApplicationBase.Start() in d:\sources\s2\Framework\Sources\UI\SharedUI\ApplicationBase.cs:line 118
   at MyApp.App1.WinUI.HDA.Main() in d:\sources\s2\App1\Sources\WinUI\HDA.cs:line 63

Answer 1

狡猾,当使用WPF,WCF和TPL的混合时,我遇到了完全相同的行为.在某些情况下,主线程的当前SynchronizationContext将变为null.

var context = SynchronizationContext.Current;

// if context is null, an exception of
// The current SynchronizationContext may not be used as a TaskScheduler.
// will be thrown
TaskScheduler.FromCurrentSynchronizationContext();

根据msdn论坛上的这篇文章,这是4.0中TPL的确认错误.同事正在4.5运行,并没有看到这种行为.

我们通过使用FromCurrentSynchronizationContext在主线程中创建一个静态单例中的TaskScheduler来解决这个问题,然后在创建continuation时始终引用该任务调度程序.例如

Task task = Task.Factory.StartNew(() =>
  {
    // something
  }
).ContinueWith(t =>
  {
    // ui stuff
  }, TheSingleton.Current.UiTaskScheduler);

这避免了.net 4.0上的TPL中的问题.

更新 如果您的开发计算机上安装了.net 4.5,即使您的目标是4.0框架,也不会看到此问题.仅安装了4.0的用户仍将受到影响.

Answer 2

不确定这是否是首选方法,但这是我如何使用SynchronizationContext:

在你的构造函数(主线程)中保存当前上下文的副本,这样你就可以保证(??)无论你在什么线程上都拥有正确的上下文.

_uiCtx = SynchronizationContext.Current;

稍后在您的任务中使用它来与主UI线程进行交互

_uiCtx.Post( ( o ) =>
{
 //UI Stuff goes here
}, null );

Answer 3

我为此创建了一个类.它看起来像这样:

public class UIContext
{
    private static TaskScheduler m_Current;

    public static TaskScheduler Current
    {
        get { return m_Current; }
        private set { m_Current = value; }
    }

    public static void Initialize()
    {
        if (Current != null)
            return;

        if (SynchronizationContext.Current == null)
            SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());

        Current = TaskScheduler.FromCurrentSynchronizationContext();
    }
}

在我的应用程序启动时,我调用UIContext.Initialize()

当我在任务中需要它时,我只需将UIContext.Current作为TaskScheduler.

Task.Factory.StartNew(() =>
{
    //Your code here
}, CancellationToken.None, TaskCreationOptions.None, UIContext.Current);