Why doesn't or operator work in if statement when using “not equals”?

Question

I am a bit confused by this. I want to hide a button if the current username of the system is != Administrator OR Administrator2, but the only way to get my desired goal is by using the && instead of || .

 string userName = System.Security.Principal.WindowsIdentity.GetCurrent().Name;

        if
            (userName != @"PC\Administrator" && userName != @"PC\Administrator2")
        {              
            button2.Hide();
        }

However, in another spot in my form to open button2 , if I use == it then seems to work?

  string userName = System.Security.Principal.WindowsIdentity.GetCurrent().Name;
  var adminform = new AdminForm();

      if  (userName == @"PC\Administrator" || userName == @"PC\Administrator2")
        {
            adminform.Show();
        }

Any idea why I can't use || when using != in the first example?

Answer 1

De Morgan's Laws for Boolean operations are an important skill for a programmer to understand intimately.

In your case, when negating you need additional parentheses to correctly order the operations, or as in your case change the || to an && . To use the || you'd put

if (!(userName == @"PC\Administrator" || userName == @"PC\Administrator2"))

Answer 2

Enclose the entire not expression in parenthesis and negate that. Like this:

if  (!(userName == @"PC\Administrator" || userName == @"PC\Administrator2"))
{
   button2.Hide();
}

Answer 3

It's the basic law of logic.

De morgan's law

!(a && b) = !a || !b

Sometimes it's easier to write it in plain words to understand:

if username is not equal to administrator OR username is not equal to administrator2 then hide the admin form

let's say that username is administrator

Then the control will hide , because one of things is true (it's not administrator2 )

Why ?

question _____________________________________value

---

if username is not equal to administrator _________0

if username is not equal to administrator2 ________1

---

Ok, so the result is 0 or 1 which is true

Answer 4

First of all, I dont really have experience with c#, but if I understood your problem correctly it is the logic you use and not ac#-only thing.

You want the method "hide" to be called when the user is not one of the two administrators, but the logic you tried was to get two boolean values: user_is_not_admin1 user_in_not_admin2

When you use && you check that the user is not admin1 and not admin 2, which works as intended. But when replacing it with || you would check that the user is never both admin1 and admin2 at the same time (which seems strange if it were to happen). Instead, when attempting to use || you also need to move the nagation. What you want is to check that the user "is not (admin1 or admin2).

For further reading, take a look at De Morgans Law. http://en.wikipedia.org/wiki/De_Morgan%27s_laws