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