c# - Nullable <bool>的null否定

Question

在否定null的bool?值的情况下，我对C#编译器感到困惑。编译器会将!null解释为null。我的期望是提高

CS0266 (Cannot implicitly convert type 'bool?' to 'bool')

样例代码:

bool? nullableVal = null;

//if (nullableVal)  //OK: CS0266 bool? can't be converted to bool
//    ;
var expectCS0266 = !nullableVal;//No compiler error/warning

//if ((!null) ?? false)//OK: CS8310 Operator '!' cannot be applied to operands of type "<NULL>"
//    ;

if (! nullableVal ?? false)
    ;//this statement isn't reached, because of precedence of ! is higher than ??
    //and !null == null

if (!(nullableVal ?? false))
    ;//this statement is reached, OK

有人可以证明为什么编译器是正确的，反之亦然。

Answer 1

参见the spec的7.3.7节:

Lifted operators permit predefined and user-defined operators that operate on non-nullable value types to also be used with nullable forms of those types. Lifted operators are constructed from predefined and user-defined operators that meet certain requirements, as described in the following:

• For the unary operators
      + ++ - -- ! ~
  a lifted form of an operator exists if the operand and result types are both non-nullable value types. The lifted form is constructed by adding a single ? modifier to the operand and result types. The lifted operator produces a null value if the operand is null. Otherwise, the lifted operator unwraps the operand, applies the underlying operator, and wraps the result.

(强调我的)

所以:

bool? x = null;
bool? y = !x;

遵守此规则。我们使用的是一元!运算符的提升形式，如果应用于的值是null，则将生成null。

禁止使用!null，因为null的类型不是Nullable<T>。 !(bool?)null有效(尽管它会产生编译器警告)。
!确实比??具有更高的优先级，请参阅第7.3.1节。