c# – 使用null-coalescing运算符进行隐式转换

public class B
{
    public static implicit operator B(A values) 
    {
        return null; 
    }
}
public class A { }

public class Program
{
    static void Main(string[] args)
    {
        A a = new A();
        B b = a ?? new B();
        //b = null ... is it wrong that I expect b to be B() ?
    }
}

此代码中的变量“b”被计算为null.我不明白为什么它是null.

我用Google搜索并在此问题中找到了回复 – Implicit casting of Null-Coalescing operator result – 与官方规范.

但是按照这个规范,我找不到“b”为空的原因:(也许我读错了,在这种情况下我为垃圾邮件道歉.

If A exists and is not a nullable type or a reference type,a compile-time error occurs.

……事实并非如此.

If b is a dynamic expression,the result type is dynamic. At run-time,a is first evaluated. If a is not null,a is converted to dynamic,and this becomes the result. Otherwise,b is evaluated,and this becomes the result.

……事实并非如此.

Otherwise,if A exists and is a nullable type and an implicit conversion exists from b to A0,the result type is A0. At run-time,a is unwrapped to type A0,b is evaluated and converted to type A0,and this becomes the result.

…存在,从b到A0的隐式转换不存在.

Otherwise,if A exists and an implicit conversion exists from b to A,the result type is A. At run-time,a becomes the result. Otherwise,b is evaluated and converted to type A,从b到A的隐式转换不存在.

Otherwise,if b has a type B and an implicit conversion exists from a to B,the result type is B. At run-time,a is unwrapped to type A0 (if A exists and is nullable) and converted to type B,b is evaluated and becomes the result.

… b有一个B型,从a到B存在隐式转换.
a被评估为null.因此,应评估b,结果应为b.

Otherwise,a and b are incompatible,and a compile-time error occurs.
Does not happen

我错过了什么吗？