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Where clause

Mai mult ca sigur cu toții am folosit clauza where.
Pe scurt, where este folosit când trebuie sa adaugăm o constrângere la un tip generic specificat la o clasa sau la o metoda.
public class Car<TType>
where TType : ICarModel
{}
In exemplul de mai sus o sa putem folosii tipuri generice TType doar dacă implementează interfața ICarModel. In cazul in care vrem sa specificam ca tipul generic sa fie neaparat o clasa este nevoie sa ne folosim si de cuvantul rezervat class:
public class Car<TType>
where TType : ICarModel, class
{}
In cazul acesta, TType trebuie sa fie obligatoriu sa fie o clasa (nu va putea sa fie o structura de exemplu). Pentru structuri trebuie sa folosim cuvantul rezervat struct:
public class Car<TType>
where TType : struct
{}
Dar dacă vrem sa face new de un obiect de tip generic? In cazul aceasta trebuie sa specificam ca are un constructor default în clasau de where. Din păcate doar constructorul default fără parametri se poate specifica. In restul cazurilor o sa aveți nevoie de un factory sau de reflection pentru a crea noi instante din interiorul clasei Car.
public class Car<TType>
where TType : new()
{}
Gata cu introducerea, sa va zic ce vroiam de fapt. Cand avem 2 clause de where sintaxa este destul de ciudata, trebuie sa specificam de doua ori where.
public class Car<TType,TFormat>
where TType : new()
where TFormat : struct
{}
Mi se pare umpic mai ciudat ca cele doua where-uri nu sunt despartite prin virgula sau alt caracter (cuvînt rezervat).
Voua cum vi se pare sintaxa?

Comments

  1. Probabil ca sa simplifice un pic parser-ul: daca permiteau ceva de genul:
    where TType: IComparable, IEnumerable, TFormat : struct
    compiler-ul ar trebui sa se uite in avand ca sa isi dea seama ca TFormat nu e un tip de baza pentru TType, ci un tip generic..

    ReplyDelete
  2. Parserele pot fi mult mai inteligente decit sintem obisnuiti. Un exemplu bun este sintaxa Razor pentru ASP.Net MVC.

    In cazul asta, insa, cred ca "where" reprezinta cuvintul rezervat. Dupa definitia clasei poate fi o acolada deschisa, un punct si virgula sau un "where".

    ReplyDelete
  3. Dar dacă vreau sa specific încă o clasa de "where" nu mai este nevoie sa pun nici o virgula sau un alt caracter special. Mi s-a părut destul de interesant, pana ieri nici nu am observat lipsa acestui semn.

    ReplyDelete

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