condition to prevent textbox null

Question

How do I make the system not allow insertion of null values inside a textbox, using id condition?

if(verificar se o textbox é null){
    MessangeBox.Show("digite um valor");
}

Answer 1

You can check whether a value is null in several ways. After all, what you consider null?

We have to consider the difference between null and emptiness, or whitespace.

Null is when a variable has no value assigned, that is, when it has never been "set". Thus, always a NullReferenceException (better known as Null Object Reference) is caused when you try to access a null object.

Emptiness is the default value of the type. For example, in a String, its default value is "", that is, empty, with no value inside. Therefore, it is not null, once it has a value in there, even though it is empty.

Whitespace is not any of the acimas, but any String with only invisible spaces and/or characters. You also enter the "Empty" category if you are looking for valid values.

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Which one will I use?

In my view of your situation, use an own . NET Framework check known as String.IsNullOrWhiteSpace. This method will verify whether a String is Null, Empty or Whitespace.

if(String.IsNullOrWhiteSpace(textBox1.Text)) {
    MessangeBox.Show("digite um valor");
    return;  // isso faz com que você saia do método e não execute os próximos processamentos
}

So if your TextBox has empty value, null or only spaces, the warning message will be displayed and will soon exit the method.

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If you want to use each method individually, both for the three elements above, do:

if(objeto == null) {
    // o objeto é nulo, não foi atribuído valor
} // ou objeto is null se a referência também for nula

if(objeto == "") {
    // o objeto não é nulo, mas é vazio
    // obs.: isso serve apenas para strings
}

if(string.IsNullOrWhiteSpace(objeto)) {
    // o objeto é nulo, vazio ou contém espaços em branco
    // obs.: isso serve apenas para strings
}