使用“不是”运算符的 C# 类型检查不起作用

Question

I'm doing this boolean check that doesn't work and i don't really get why.

I have a variable: dynamic value = 10f; This variable is of type float . Now i'm doing this check:

if(value is not float || value is not int)
{
    // Execute code
}

But this code still executes. Can anyone please explain?

Answer 1

I found the solution thanks to the comment section. So i changed if (value is not BuiltInClass || value is not ClassValue) to if (!(value is BuiltInClass || value is ClassValue)) and now it works.

Answer 2

You have misunderstood the boolean expression.

Suppose value is of type BuiltInClass :

Then

• value is not BuiltInClass will be false.
• value is not ClassValue will be true.

Thus

value is not BuiltInClass || value is not ClassValue

resolves to

false || true => true

You "fixed" this by using if (!(value is BuiltInClass || value is ClassValue)) instead, but that is NOT equivalent to your original code!

It would be equivalent to this:

if (value is not BuiltInClass && value is not ClassValue) 

Note the use of && instead of || .

For more information about boolean transforms, see De Morgan's laws .

In particular, note the transformation:

not (A or B) = (not A) and (not B)

Your fix is not (A or B) , but you could have fixed it by changing it to (not A) and (not B) as per De Morgan's laws.