c# - 为什么空传播对事件不起作用？

Question

背景:C# : The New and Improved C# 6.0

using System;

internal sealed class Program
{
    private sealed class Inner
    {
        internal int Value { get; } = 42;
        internal void DoSomething(int value) { }
        internal event EventHandler Event;
    }

    private sealed class Outer
    {
        internal Inner Inner { get; } = new Inner();
    }

    private static void Main(string[] args)
    {
        Outer outer = null;

        // Works as expected (does not call Inner and Value, val is null)
        int? val = outer?.Inner.Value;

        // Works as expected (does not call Inner and DoSomething)
        outer?.Inner.DoSomething(42);

        // CS0070: The event 'Program.Inner.Event' can only appear on the left hand
        // side of += or -= (except when used from within the type 'Program.Inner')
        outer?.Inner.Event += (s, e) => { };
    }
}

由于 += 运算符只是调用事件的 add 方法的语法糖，我本以为最后一行编译就像调用 DoSomething()(并且它在运行时不执行任何操作)。

Answer 1

+= 运算符确实是方法调用的语法糖，但它是运算符，而不是方法调用。

+= 运算符左侧的代码是:

outer?.Inner.Event

该运算符左侧的代码需要评估可以分配给并定义了 + 运算符的东西(例如委托(delegate)类型的变量)或事件。

如果 outer == null，此代码无法评估为事件，这就是它非法的原因。