c# - 比较两个可空值

Question

C# 5.0 规范在第 7.1.3 章中读取

https://msdn.microsoft.com/en-us/library/ms228593.aspx

The lifted operator produces the value false if one or both operands are null.

但是测试以及这个 MSDN 链接

http://msdn.microsoft.com/en-us/library/2cf62fcy(v=vs.100).aspx

int? num1 = 10;
int? num2 = null;

// Change the value of num1, so that both num1 and num2 are null.
num1 = null;
if (num1 == num2)
{
  // The equality comparison returns true when both operands are null.
  Console.WriteLine("num1 == num2 returns true when the value of each is null");
}

/* Output:
 * num1 == num2 returns true when the value of each is null
 */

显示比较两个均为 null 的可空值返回 true。

这是有道理的，但它与规范中的句子相矛盾，不是吗？

Answer 1

不要混用，这是关于不同类型的运算符。

• For the equality operators == != a lifted form of an operator exists if the operand types are both non-nullable value types and if the result type is bool. The lifted form is constructed by adding a single ? modifier to each operand type. The lifted operator considers two null values equal, and a null value unequal to any non-null value. If both operands are non-null, the lifted operator unwraps the operands and applies the underlying operator to produce the bool result.

• For the relational operators < > <= >= a lifted form of an operator exists if the operand types are both non-nullable value types and if the result type is bool. The lifted form is constructed by adding a single ? modifier to each operand type. The lifted operator produces the value false if one or both operands are null. Otherwise, the lifted operator unwraps the operands and applies the underlying operator to produce the bool result.