c# - 分配 bool 值？ bool

Question

考虑以下代码:

bool x;
bool? y = null;
x = y?? true;

将 bool? 分配给 bool 是一个编译时错误，但上面的代码在编译和运行时都成功了。为什么？尽管第三条语句确保我们永远不会将 null 分配给 x，但如果 y 不为 null，我们仍然会分配一个 bool ? 到 bool，所以它应该是编译器 POV 的错误，不是吗？

还是 C# 编译器足够聪明，可以发现一段特定的代码不可能创建将 null 分配给 x 的情况？

Answer 1

这个表达式的类型:

y ?? true

是bool，不是bool?。

来自 C# 5 规范的第 7.13 节:

The type of the expression a ?? b depends on which implicit conversions are available on the operands. In order of preference, the type of a ?? b is A₀, A, or B, where A is the type of a (provided that a has a type), B is the type of b (provided that b has a type), and A₀ is the underlying type of A if A is a nullable type, or A otherwise. Specifically, a ?? b is processed as follows:

• If A exists and is not a nullable type or a reference type, a compile-time error occurs.
• 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.
• 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 first evaluated. If a is not null, a is unwrapped to type A₀, and this becomes the result. Otherwise, b is evaluated and converted to type A₀, and this becomes the result.
• Otherwise, if A exists and an implicit conversion exists from b to A, the result type is A. At run-time, a is first evaluated. If a is not null, a becomes the result. Otherwise, b is evaluated and converted to type A, and this becomes the result.
• 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 first evaluated. If a is not null, a is unwrapped to type A₀ (if A exists and is nullable) and converted to type B, and this becomes the result. Otherwise, b is evaluated and becomes the result.
• Otherwise, a and b are incompatible, and a compile-time error occurs.

对于您的情况，我们在第三个项目符号中:

• A 是 bool?
• A₀ 是 bool
• B 是bool

...所以结果类型是 bool，您可以将其分配给 bool 类型的变量。