用于单元测试事件是否已被订阅的扩展方法

public static class EventExtensions
{
    public static bool IsSubscribed<T>(this EventHandler<T> eventHandler) where T : EventArgs
    {
        return eventHandler != null && eventHandler.GetInvocationList().Length > 0;
    }
}

public class EventPublisher
{
    public event EventHandler<EventArgs> MyEvent;
}

public class EventSubscriber
{
    public void Subscribe(EventPublisher publisher)
    {
        publisher.MyEvent += HandleEvent;
    }

    public void Unsubscribe(EventPublisher publisher)
    {
        publisher.MyEvent -= HandleEvent;
    }

    private void HandleEvent(object sender, EventArgs e)
    {
        // 处理事件的逻辑
    }
}

public class UnitTest
{
    [Fact]
    public void TestEventSubscription()
    {
        var publisher = new EventPublisher();
        var subscriber = new EventSubscriber();

        Assert.False(publisher.MyEvent.IsSubscribed()); // 检查事件是否已被订阅

        subscriber.Subscribe(publisher);

        Assert.True(publisher.MyEvent.IsSubscribed()); // 检查事件是否已被订阅

        subscriber.Unsubscribe(publisher);

        Assert.False(publisher.MyEvent.IsSubscribed()); // 检查事件是否已被订阅
    }
}