Why does it compile?

Question

            var cv = (new CustomValidator()
            {
                 ErrorMessage="Formato de telefone inválido",
                 ControlToValidate = "txtTelefoneIni", //<-- see the extra comma
            });

Answer 1

Because it is legal syntax.

Really, it is.

When you're constructing an object using the object initializer syntax, you're allowed to leave a trailing comma after the last item, even if you end the initializer block there and then.

The reason behind it is probably that it makes it easier to come back and edit it later, as forgetting to add a comma after the previously-last-item is the #1 compile-time-problem with object-initializers.