Question

我的应用程序出了问题：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，这些都是可能与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，在我的真实应用程序中，执行延续任务时，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？

修改

@Hans要求堆栈跟踪。这是：


   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

Sly，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将变为空。

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this post，这是4.0中TPL中确认的错误。同事正在4.5上运行，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。

<强>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。只安装了4.0的用户仍然会受到影响。

Answer 2

不确定这是否是首选方法，但以下是我使用SynchronizationContext的方法：

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有正确的上下文。

_uiCtx = SynchronizationContext.Current;

稍后在您的任务中使用它与主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();
    }
}

在启动我的应用程序时，我调用UIContext.Initialize（）

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

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

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

3 个答案: