RegisterClassAlias with custom objects

[Dave81]

I have defined a custom class:

public class ClasseProva
{
private var numero:int;
public function ClasseProva(){}
public function setNumber(num:int):void
{
this.numero = num;
}

public function getNumber():int
{
return this.numero;
}
}


Then, out of this class, two functions:

public function scrivi():void
{
var classeProva:ClasseProva = new ClasseProva();
classeProva.setNumber(5);

registerClassAlias("ClasseProvaAlias", ClasseProva);

var file:File = File.applicationStorageDirectory;
file = file.resolvePath(nomeFile);

var fileStream:FileStream = new FileStream();

fileStream.open(file, FileMode.WRITE);
fileStream.writeObject(classeProva);
fileStream.close();
trace("Numero: " + classeProva.getNumber().toString());
}

public function leggi():void
{
getClassByAlias("ClasseProvaAlias");
var file:File = File.applicationStorageDirectory;
file = file.resolvePath(nomeFile);

var fileStream:FileStream = new FileStream();

fileStream.open(file, FileMode.READ);
var classeProva:ClasseProva = fileStream.readObject();

fileStream.close();
trace("Numero: " + classeProva.getNumber().toString());
}


After calling the "scrivi" (write) function, I call the "leggi" (read) function... the number printed with trace(...) is always 0. Can somebody help me?