字符串比较中的逻辑OR运算符？

Question

Why does this comparison fails? How do I compare for alternating strings in csharp ?

Static void Main(string[] args)
    {
        string varFoo = "cat";

        if (varFoo != "cat" || varFoo!="duck")
            Console.WriteLine("You can enter.");
        else
            Console.WriteLine("Not allowed.");

        Console.ReadLine();
    }

Just wanted something as

If(Either cat or a Duck)
    // You're not allowed
else
   // you are welcomed.

Answer 1

If I understand your comments, "cat" is allowed (and I suppose "duck" is also allowed).

if (varFoo == "cat" || varFoo=="duck")

With your last edit (cat and duck can't enter).

if (varFoo != "cat" && varFoo !="duck")

this means : if varFoo == "cat" : assertion will fail (left part is evaluated, as it is false, right part isn't evaluated => false).

if varFoo == "duck" : assertion will fail (left part is evaluated, it's true, then right part is evaluated, it's false => false)

this is just boolean way of life :

true or false => true
true or true => true
true and false => false
false and true => false

Answer 2

One of your conditions is always true, no matter what the actual value is. You probably wanted a different comparison.

Considering the updated question, you'd be looking for the expression varFoo != "cat" && varFoo != "duck" , as you obviously confused && (both conditions have to be true) and || (either condition have to be true).

Answer 3

From your comment

Q: in what way does it fail? – dutzu

A: The cat is allowed ._.

i assume that you want to keap away cats and ducks.

You condition fails because of the Logical OR Operator || . The second condition gets evaluated because "cat" != "cat" returns false but "cat" != "duck" returns true. That's why cats are also allowed to enter.

You probably want to stop both from entering with

if (varFoo != "cat" && varFoo != "duck")
    Console.WriteLine("You can enter.");
else
    Console.WriteLine("Not allowed.");

Update You last edit supports my opinion:

Just wanted something as

If(Either cat or a Duck)
    // You're not allowed

Understand it differently: You define an action, this is to prevent some animals from entering. You want to apply this action on cats and ducks . You need an AND instead of an OR . (see above code)

Another approach which does the same is to define a collection of forbidden animals and use Contains :

IEnumerable<string> forbiddenAnimals = new List<string>(){ "cat", "duck" };
if(forbiddenAnimals.Contains(varFoo))
    // You're not allowed
else
    // you are welcomed

Answer 4

if (varFoo != "cat" || varFoo!="duck")

translation:

varFoo is not equal to cat OR varFoo is not equal to duck . Since varFoo is not duck ; it returns true.

Answer 5

You probably want this:

if (varfoo != "cat" && varfoo != "duck")
  //You can enter.

With the || it will always fail, on all strings.

Answer 6

if(varFoo != "cat" || varFoo != "duck") so if( false || true ) false or true == true. it doesn't fail.