SynchronizationContext 和 DispatcherUnhandledException

Question

我遇到了这样一种情况，即 UI 线程中抛出的异常没有在调用线程中被捕获。

using System;
using System.Diagnostics;
using System.Threading;
using System.Windows;

namespace SynchronisationContextAndExceptionWPF
{
    public partial class MainWindow : Window
    {
        private readonly SynchronizationContext _synchronizationContext;

        public MainWindow()
        {
            InitializeComponent();

            _synchronizationContext = SynchronizationContext.Current;
        }


        private void Button_OnClick(object sender, RoutedEventArgs e)
        {
            try
            {
                _synchronizationContext.Send(
                    x =>
                    {
                        try
                        {
                            DoSomethingOnUiThreadThatThrowsException();
                        }
                        catch (Exception)
                        {
                            Debug.WriteLine("Catched Exception in thread that threw it.");
                            throw;
                        }
                    }, null);
            }
            catch (Exception)
            {
                Debug.WriteLine("Catched Exception in thread that calles Send-Method.");
                throw;
            }
        }

        private static void DoSomethingOnUiThreadThatThrowsException()
        {
            throw new Exception("Any Exception...");
        }
    }
}

首先我认为这是不可能的（我发现的所有文档都说我可以在那里捕获异常）。 经过一番研究，我发现了问题：我的应用程序使用了 UnhandledExceptionHandler。处理DispatcherUnhandledException-Event。我正在向用户显示一些信息并设置e.Handled = true;：

using System.Diagnostics;
using System.Windows;
using System.Windows.Threading;

namespace SynchronisationContextAndExceptionWPF
{
    public partial class App : Application
    {
        public App()
        {
            DispatcherUnhandledException += App_DispatcherUnhandledException;
        }

        private static void App_DispatcherUnhandledException(
            object sender,
            DispatcherUnhandledExceptionEventArgs e)
        {
            Debug.WriteLine("Catched Exception in UnhandledExceptionHandler.");

            // This line makes the difference:
            e.Handled = true;
        }
    }
}

所以问题是：为什么即使我处理了DispatcherUnhandledException-Event 也会引发它？ 您将如何解决这种情况？

Answer 1

如果你有很多控件，你可以生成一个新的类来记住特殊的异常变量。因此，您只需要更改 _synchronizationContext 的初始化（希望只在控件的基类中更改一次）。

public partial class MainWindow : Window
{
    private readonly MySynchronizationContext _synchronizationContext;

    public MainWindow()
    {
        InitializeComponent();

        _synchronizationContext = new MySynchronizationContext(SynchronizationContext.Current);

    }

    private void button_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            _synchronizationContext.Send(
                x =>
                {
                    DoSomethingOnUiThreadThatThrowsException();
                }, null);
        }
        catch (Exception)
        {
            Debug.WriteLine("Catched Exception in thread that calles Send-Method.");
            throw;
        }
    }

    private static void DoSomethingOnUiThreadThatThrowsException()
    {
        throw new Exception("Any Exception...");
    }
}

class MySynchronizationContext
{
    SynchronizationContext innerContext;

    public MySynchronizationContext(SynchronizationContext ctx)
    {
        innerContext = ctx;
    }

    public virtual void Send(SendOrPostCallback d, object state)
    {
        Exception threadException = null;

        try
        {
            innerContext.Send(_ =>
            {
                try
                {
                    d.Invoke(state);
                }
                catch (Exception exception)
                {
                    threadException = exception;
                }
            }, null);
        }
        catch (Exception ex)
        {

        }

        if (threadException != null)
        {
            throw new Exception("Synchronization error", threadException);
        }
    }
}

Answer 2

在您的 lambda 表达式中，您可以设置一个异常变量并稍后在调用线程中检查此变量。如果已设置，则在调用线程时抛出异常。

private void button_Click(object sender, RoutedEventArgs e)
    {
        Exception threadException = null;

        try
        {
            _synchronizationContext.Send(
                x =>
                {
                    try
                    {
                        DoSomethingOnUiThreadThatThrowsException();
                    }
                    catch (Exception ex)
                    {
                        Debug.WriteLine("Catched Exception in thread that threw it.");
                        threadException = ex;
                        //throw; --> don't throw exception here; otherwise you will get DispatcherUnhandledException twice.
                    }
                }, null);
        }
        catch (Exception)
        {
            Debug.WriteLine("Catched Exception in thread that calles Send-Method.");
            throw;
        }

        if(threadException != null)
        {
            Debug.WriteLine("Catched Exception in thread that calles Send-Method.");
            throw threadException; //throw you previously catched exception here.
        }
    }

亲切的问候， 丹尼尔