c# - 成员引用和评估的分配顺序

Question

为什么以下语句会因 NullReferenceException 而崩溃 a.b.c = LazyInitBAndReturnValue(a);？

class A {
    public B b;
}

class B {
    public int c;
    public int other, various, fields;
}

class Program {

    private static int LazyInitBAndReturnValue(A a)
    {
        if (a.b == null)
            a.b = new B();

        return 42;
    }

    static void Main(string[] args)
    {
        A a = new A();
        a.b.c = LazyInitBAndReturnValue(a);
        a.b.other = LazyInitBAndReturnValue(a);
        a.b.various = LazyInitBAndReturnValue(a);
        a.b.fields = LazyInitBAndReturnValue(a);
    }
}

赋值表达式被求值from right to left ，因此当我们分配给 a.b.c 时，a.b 不应为空。奇怪的是，当调试器因异常而中断时，它也将 a.b 显示为已初始化为非空值。

Answer 1

这在 Section 7.13.1 中有详细说明C# 规范。

The run-time processing of a simple assignment of the form x = y consists of the following steps:

• If x is classified as a variable:
  • x is evaluated to produce the variable.
  • y is evaluated and, if required, converted to the type of x through an implicit conversion (Section 6.1).
  • If the variable given by x is an array element of a reference-type, a run-time check is performed to ensure that the value computed for y is compatible with the array instance of which x is an element. The check succeeds if y is null, or if an implicit reference conversion (Section 6.1.4) exists from the actual type of the instance referenced by y to the actual element type of the array instance containing x. Otherwise, a System.ArrayTypeMismatchException is thrown.
  • The value resulting from the evaluation and conversion of y is stored into the location given by the evaluation of x.
• If x is classified as a property or indexer access:
  • The instance expression (if x is not static) and the argument list (if x is an indexer access) associated with x are evaluated, and the results are used in the subsequent set accessor invocation.
  • y is evaluated and, if required, converted to the type of x through an implicit conversion (Section 6.1).
  • The set accessor of x is invoked with the value computed for y as its value argument.

我认为底部部分(如果 x 被分类为属性或索引器访问)提供了一个提示，但也许 C# 专家可以澄清。

首先生成一个set accessor，然后评估y(触发你的断点)，然后调用set accessor，这导致空引用异常。如果非要我猜的话，我会说 accessor 指向 b 的旧值，它是 null。当您更新 b 时，它不会更新它已经创建的访问器。