1. 问题

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        ContainerLocator.Container.Resolve<TestViewModel>();
    }
}
public class TestViewModel
{
    public TestViewModel(IEventAggregator eventAggregator)
    {
        var testEvent = eventAggregator.GetEvent<TestEvent>();
        testEvent.Subscribe(() => { }, ThreadOption.UIThread);
    }
}
public class TestEvent : PubSubEvent
{
}

上面是一段使用了 Prism 的单元测试，它主要的逻辑是在 EventAggregator 中订阅了 TestEvent，当接收到消息后在 UI 线程上执行后续的逻辑。这种代码在正常程序中没有问题，但在单元测试中会报错：

System.InvalidOperationException: To use the UIThread option for subscribing, the EventAggregator must be constructed on the UI thread.

2. 原因

翻翻源码，可以发现这个 Exception 在 PubSubEvent 的 Subscribe 函数中抛出：

switch (threadOption)
{
    case ThreadOption.PublisherThread:
        subscription = new EventSubscription(actionReference);
        break;
    case ThreadOption.BackgroundThread:
        subscription = new BackgroundEventSubscription(actionReference);
        break;
    case ThreadOption.UIThread:
        if (SynchronizationContext == null) throw new InvalidOperationException(Resources.EventAggregatorNotConstructedOnUIThread);
        subscription = new DispatcherEventSubscription(actionReference, SynchronizationContext);
        break;
    default:
        subscription = new EventSubscription(actionReference);
        break;

当 SynchronizationContext 为 null 时就会判断当前不在 UI 线程，然后抛出 Exception。而 SynchronizationContext 又是在 EventAggregator 中赋值：

private readonly SynchronizationContext syncContext = SynchronizationContext.Current;
public TEventType GetEvent<TEventType>() where TEventType : EventBase, new()
{
    lock (events)
    {
        EventBase existingEvent = null;
        if (!events.TryGetValue(typeof(TEventType), out existingEvent))
        {
            TEventType newEvent = new TEventType();
            newEvent.SynchronizationContext = syncContext;
            events[typeof(TEventType)] = newEvent;
            return newEvent;
        }
        else
        {
            return (TEventType)existingEvent;
        }
    }
}

问题就出在 SynchronizationContext.Current 这里。这个属性用于获取当前线程的同步上下文。不是每一个线程都有一个 SynchronizationContext 对象。一个总是有 SynchronizationContext 对象的是UI线程。由于单元测试并不是运行在 UI 线程，所以这个属性在单元测试中一直为 null。

3. 解决方案

现在我们知道问题原因了，解决方案也很简单，只要自定义一个 EventAggregator，源码全部照抄，但是把这句：

private readonly SynchronizationContext syncContext = SynchronizationContext.Current;

替换成这句：

private readonly SynchronizationContext syncContext = new SynchronizationContext();

就不会出现 PubSubEvent 中 SynchronizationContext 等于 null 的情况了。然后再把这个类注册到容器中作为 IEventAggregator：

ContainerLocator.Current.RegisterSingleton<IEventAggregator, MyEventAggregator>();

4. 最后

根据单元测试项目的结构，容器的初始化会有不同的方式，如果想尽量模仿 PrismApplication 的话可以参考 PrismApplicationBase 和 PrismInitializationExtensions 写一个初始化类，大概差不多这样（简化了部分代码）：

[TestClass]
public abstract class TestInitializerBase
{
    public void Initialize()
    {
        ContainerLocator.SetContainerExtension(() => new UnityContainerExtension());
        ContainerExtension = ContainerLocator.Current;
        ContainerExtension.RegisterSingleton<IDialogService, DialogService>();
        ContainerExtension.RegisterSingleton<IModuleInitializer, ModuleInitializer>();
        ContainerExtension.RegisterSingleton<IModuleManager, ModuleManager>();
        ContainerExtension.RegisterSingleton<RegionAdapterMappings>();
        ContainerExtension.RegisterSingleton<IRegionManager, RegionManager>();
        ContainerExtension.RegisterSingleton<IRegionNavigationContentLoader, RegionNavigationContentLoader>();
        ContainerExtension.RegisterSingleton<IEventAggregator, EventAggregator>();
        ContainerExtension.RegisterSingleton<IRegionViewRegistry, RegionViewRegistry>();
        ContainerExtension.RegisterSingleton<IRegionBehaviorFactory, RegionBehaviorFactory>();
        ContainerExtension.Register<IRegionNavigationJournalEntry, RegionNavigationJournalEntry>();
        ContainerExtension.Register<IRegionNavigationJournal, RegionNavigationJournal>();
        ContainerExtension.Register<IRegionNavigationService, RegionNavigationService>();
      
        RegisterRequiredTypes(ContainerExtension);
    }
    public IContainerExtension ContainerExtension { get; private set; }
    protected abstract void RegisterRequiredTypes(IContainerRegistry containerRegistry);
}
public class TestInitializer : TestInitializerBase
{
    [AssemblyInitialize]
    public static void InitializeAseemble(TestContext testContext)
    {
        var testInitializer = new TestInitializer();
        testInitializer.Initialize();
    }
    protected override void RegisterRequiredTypes(IContainerRegistry containerRegistry)
    {
        containerRegistry.RegisterSingleton<IEventAggregator, MyEventAggregator>();
    }
}

这样在 TestInitializer 中可以注册各种方便单元测试的伪对象。