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How NOT to use AS keyword

Mai mult ca sigur ati folosit pana acuma "as". Prin intermediul sau se poate face cast unui obiect la un tip dat, iar in cazul in care cast-ul nu se poate face, valoarea returnata este null.
BaseItem item = val as BaseItem;    // returneaza null daca nu poate sa faca conversia
BaseItem item = (BaseItem) val; // arunca exceptie daca nu poate sa faca conversia
Despre "as" am mai discutat in acest post: http://vunvulearadu.blogspot.com/search/label/C%23%20AS%20Keyword%20Cast%20speed%20performance
Mai jos o sa va prezint un mod mai ciudat de a folosii keyword-ul "as". A fost folosit pentru a detecta daca un element este de un anumit. Iata cum a fost folosit:
BaseItem item;
...
if ( item as CarItem != null )
{
...
}
else if ( item as BookItem != null )
{
...
}
else if ( item as ClassItem != null )
{
...
}
Ce mi s-a parut interesant este cum a fost folosit "as". In locul sau se poate folosii fara probleme "is". Acesta o sa returneze true daca un obiect se poate converti la tipul dat. Codul de mai sus se poate rescrie in felul urmtor:
BaseItem item;
...
if ( item is CarItem )
{
...
}
else if ( item is BookItem )
{
...
}
else if ( item is ClassItem )
{
...
}
Codul arata ceva mai bine, dar inca ceva ce nu pare okay. Ca sa scapam de acest lant de IF-uri putem sa ne declaram in BaseItem o metoda virtuala sau abstracta la care sa facem override in clasele noastre( CarItem, BookItem,CLassItem). Prin acest mod am avea un singur apel la o metoda, care pentru fiecare din clasele noastre poate sa execute codul dorit:
abstract  class BaseItem
{
public abstract void DoAction();
}
public class CarItem : BaseItem
{
public override void DoAction()
{
...
}
}
... pentru fiecare din clasele noastre ...
Iar insiruirea de IF-uri pe care era mai sus devine:
item.DoAction();

Enjoy!

Comments

  1. Cel ce a scris codul ala, n-o fi auzit de 'is' sau polimorfism, se mai intampla.. :)

    ReplyDelete
  2. Si ce faci daca nu ai control asupra clasei respective? Primesti un object si in functie de tipul lui real trebuie sa procedezi intr-un fel? Ai posibilitatea Double Dispatch, care este un pattern consacrat, dar de asemenea (in .NET 4.0) la ceva de genul:
    myMethod((dynamic)obj);
    ca apoi sa implementezi myMethod pentru diferite tipuri. "Defaultul" fiind myMethod(object obj) { throw something }

    ReplyDelete
  3. Belele si mai mari poti avea cu "as" cand nu se face verificarea de null a rezultatului si se paseaza acest rezultat unei alte metode si care il mai paseaza altei metode si intr-un final crapa cu NullReferenceException dar nu unde a esuat conversia ci zece kilometri mai departe, ingreunand depanarea.

    Nu ma intrebati cum stiu asta...

    ReplyDelete
    Replies
    1. Odata si odata tot apare un NULL, daca nu faci o verificare dupa ce faci conversia sau cand referi variabila obtinuta cu "as". Nu te intreb de unde stii, presupun ca ai avut parte de un debug destul de naspa :-(

      Delete

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