c# - 为什么在分配方法组时不引发 "forwarded"事件，但在分配委托(delegate)时引发？

Question

给定以下代码:

public delegate void Signal();

public static class SignalExtensions
{
    public static void SafeInvoke(this Signal signal)
    {
        Signal copy = signal;
        if (copy != null)
        {
            copy();
        }
    }
}

public class RootEventSource
{
    public event Signal RootEvent;

    public void Raise()
    {
        this.RootEvent.SafeInvoke();
    }
}

public class EventForwarder
{
    private readonly RootEventSource rootEventSource;

    public EventForwarder(RootEventSource rootEventSource)
    {
        this.rootEventSource = rootEventSource;

        // this is the critical part
        this.rootEventSource.RootEvent 
             += () => this.AnotherEvent.SafeInvoke();
    }

    public event Signal AnotherEvent;

    // just an example of another method which is using the root event source
    public override string ToString()
    {
        return this.rootEventSource.ToString();
    }
}

class Program
{
    static void Main(string[] args)
    {
        var rootEventSource = new RootEventSource();
        var eventForwarder = new EventForwarder(rootEventSource);

        eventForwarder.AnotherEvent += HandleAnotherEvent;

        rootEventSource.Raise();

        Console.WriteLine("done");
        Console.ReadKey();
    }

    private static void HandleAnotherEvent()
    {
        Console.WriteLine("received AnotherEvent");
    }
}

这导致输出:

received AnotherEvent
done

---

现在我对 EventForwarder 的实现稍作改动，使用一个方法组来转发事件:

public EventForwarder(RootEventSource rootEventSource)
{
    this.rootEventSource = rootEventSource;
    this.rootEventSource.RootEvent += this.AnotherEvent.SafeInvoke;
}

输出变为:

done

因此 AnotherEvent 未引发。

---

到现在为止我会考虑这两行:

this.rootEventSource.RootEvent += this.AnotherEvent.SafeInvoke;
this.rootEventSource.RootEvent += () => this.AnotherEvent.SafeInvoke();

等价。看起来他们不是。

那么有什么区别呢？另外，为什么没有引发该事件？

---

PS:虽然 R# 通常建议将 () => this.AnotherEvent.SafeInvoke(); 替换为 this.AnotherEvent.SafeInvoke 但它并没有这样做这里。所以显然它知道它不应该在这里这样做。

Answer 1

当您将方法组分配给这样的事件时:

this.rootEventSource.RootEvent += this.AnotherEvent.SafeInvoke;

您实际上从 SignalExtensions.SafeInvoke 方法创建了一个委托(delegate)，它作为参数接受您的 this.AnotherEvent 委托(delegate)对象。由于它最初是 null，因此您创建了一个带有 null 参数的委托(delegate)。这个 null 值当然永远不会改变，因为委托(delegate)是不可变的。

如果你想转发一个事件你应该这样做:

public class EventForwarder
{
    private readonly RootEventSource rootEventSource;

    public EventForwarder(RootEventSource rootEventSource)
    {
        this.rootEventSource = rootEventSource;
    }

    public event Signal AnotherEvent
    {
        add { this.rootEventSource.RootEvent += value; }
        remove { this.rootEventSource.RootEvent -= value; }
    }
}