该方法返回的是真还是假？

Question

bool isValid = false;
string username = "someadmin";

If( !String.IsNullOrEmpty(username) && !( username.IndexOf("admin") != -1)
    isValid = true;

The second part with the double negatives is crossing me up!

Answer 1

May I present to you DeMorgan's Laws :

NOT (P OR Q) = (NOT P) AND (NOT Q)
NOT (P AND Q) = (NOT P) OR (NOT Q)

So, you could rewrite it as:

if (!(String.IsNullOrEmpty(username) || username.IndexOf("admin") != -1)) {
    isValid = true;
}

...thus removing the double negatives.

Furthermore, you could say:

if (String.IsNullOrEmpty(username) || username.IndexOf("admin") != -1) {
    isValid = false;
}

...which removes all the negatives.

Also, you could say:

isValid = !(String.IsNullOrEmpty(username) || username.IndexOf("admin") != -1));

...to make it nice and compact.

Answer 2

it will return false

!String.IsNullOrEmpty(username)          // this is true, the string is not NullOrEmpty
!(username.IndexOf("admin") != -1)       // IndexOf is >= 0, so != 1 is true. But the first ! makes it false

So IsValid will contain the same value as it had at the beginning...

Answer 3

它将返回false。

Answer 4

A plain language version:

if (username is not null or empty and username doesn't contain "admin") isValid = true;

Answer 5

isValid will be false.

(Nitpick: this code doesn't "return" anything: it just sets the value of the isValid variable.)

Answer 6

false. But I don't get the question? Couldn't you just execute this? What double negatives? The value is just being inverted and the parentheses clearly indicate the order in which the statements are executed.

Answer 7

存储在isValid的值将为false 。

Answer 8

It will give you a syntax error due to a missing paren ;-)

But seriously, it will return false.

!String.IsNullOrEmpty(username) // if username is NOT null and NOT empty => true

username.IndexOf("admin") != -1 // if "admin" IS FOUND in username (not -1) => true

!( username.IndexOf("admin") != -1 ) // if "admin" IS NOT FOUND in username => false

So, concluding: the conditions of the if statement are NOT met, so isValid will remain false.

PS.: I'm not ac# programmer, but I presume that when IndexOf(string) equals -1, it means not found.