What does "!" (exclamation) mean before a code snippet?

Question

There are many basic things I do not know, for example how to read exactly this exclamation the way it is placed in the following section:

if (!(periodoAnterior.Equals(dataReader["NOMEPERIODO"].ToString()))){
    //...
}

Can anyone explain to me? And taking advantage, this Equals does the same as "==" ?

Answer 1

! is the negation operator. It returns the opposite of the resolution of the operation which it precedes.

That is to say:

!true == false
!false == true
!(2 == 2) == false
!(2 == 1) == true

Answer 2

Anything you put inside a if(aqui) will be transformed into true or false. The body of if only executes if the result is true.

Let’s simplify your example by keeping the result of Equals in a variable:

bool resultado = periodoAnterior.Equals(dataReader["NOMEPERIODO"].ToString());

The variable resultado contains true or false. So we have two options for the if:

if (!(true)) {}
if (!(false)) {}

What is the exclamation ! does is invert the value of a boolean: !true (read "not true") is false, and !false is true. Therefore, the possible results for the if evening:

if(false) {} // aqui o corpo nunca executa
if(true) {} // aqui o corpo executa

Thus, in its code, the body of if only executes if the result of periodoAnterior.Equals(dataReader["NOMEPERIODO"].ToString()) for false.

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^{Note: Your doubt as to Equals would deserve a separate question, but probably who wrote it led to C# a Java addiction. More details and exceptions in https://stackoverflow.com/questions/1659097/c-string-equals-vs}