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ReadOnlyCollection

Va mai aduceti aminte ca in C++ puteam sa declaram o variabila sa fie constanta, fara a ne face probleme ca un utilizator ii poate schimba valoarea. In C# putem sa facem un lucru asemanator folosindu-ne de readonly.
public class Car
{
internal readonly int _id;

public Car(int id)
{
_id = id;
}
}
Doar in contructor se va putea initializa field-ul _id. Valoarea acestui camp nu se va putea modifica in nici o locatie din cod. Dar daca in loc de int am aveam o lista, ar aparea probleme. Chiar daca nu se va putea instanta din nou acest camp, continutul acestuia se va putea modifica.
public class Car
{
internal readonly List<Component> _components;

public Car(List<Component> components)
{
_components = components;
}
}

public class Logan : Car
{
// ...

public void SomeMethod()
{
_components.Add(..);
_components.Remove(..);
}
}
Oricine ar putea sa adauge sau sa stearga elemente din lista. Am avea nevoie de o lista care sa permita doar operatii de read. Pentru acest lucru .NET ne vine in ajutor si ne ofera clasa generica ReadOnlyCollection. Aceasta colectie nu contine metode prin care putem sa adaugam elemente sau sa stergem elemente din lista. Totodata indexer-ul ne permite sa facem doar get si atat.
Orice lista contine metoda AsReadOnly(), care ne returneaza o lista de tip ReadOnlyCollection.
List<int> items = new List<int>(){ 1, 2 ,3};
items.Add(4);
items.Remove(1);
items[1] = 10;
ReadOnlyCollection<int> itemsReadOnly = items.AsReadOnly();
itemsReadOnly.Add(4); //Eroare la compilare, metoda nu exista;
itemsReadOnly.Remove(1); //Eroare la compilare, metoda nu exista;
itemsReadOnly[1] = 10; //Doar get se poate face;
Chiar daca incercam sa facem apoi o conversie la List, nu o sa putem, .NET ne returneaza o eroare de genul: collection is read-only.
Exemplul dat cu clasa de tip Car, poate sa rescris in forma urmatoare:
public class Car
{
internal readonly ReadOnlyCollection<Component> _components;

public Car(ReadOnlyCollection<Component> components) //sau public Car(List<Component> components)
{
_components = components; //sau _components = components.AsReadOnly();
}
}

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