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Dictionary - custom key type

In ultimele 2 saptamani am luat la puricat .NET Collections si m-am gandit ca merita sa vorbim astazi putin despre cheile unui dictionar( Dictionary) - map.
In general lucram cu chei care sunt de tip value type.

Dictionary<int, Door> map = new Dictionary<int, Door>();
map.Add(1,door1);
map.Add(2,door2);
...
var door1 = map[1];
In acest caz, cheia o sa fie gasita fara nici o problema. Dar ce se intampla cand cheia noastra este reprezentata de un tip declarat de noi?
Dictionary<DoorKey,Door> map = new Dictionary<DoorKey, Door>();
map.Add(doorKey1, door1);
map.Add(doorKey2, door2);
....
var door1 = map[copyOfDoorKey1];
In momentul cand o sa dorim sa facem retrive la element pe baza de cheie, o sa observam ca elementul nu este gasit( o eroare de tip KeyNotFoundException o sa fie aruncata). In debug daca ne uitam la ce elemente contine dictionarul, o sa observam ca avem doua elemente cu cheiule pe care le-am astepta, dar totusi nu putem sa facem retrive la date.
Fiecare colectie de tip (cheie,valoare) are implementari diferete, dar in general una sau mai multe elemente sunt grupate intr-un bucket( galeata), pe baza la hash code. Dimensiunea sau distributia pe valori a acestuia difera, dar ce trebuie sa retinem aici este ca fiecare element este adaugat intr-un anumit bucket pe baza unui hash returnat de catre cheie. Dupa ce este identificat bucket-ul, se face o cautare a cheii in bucket-ul respectiv pe baza metodei Equal.
Indiferent de metoda de implementare a dictionarului, cautarea se face pe baza acestor doua metode. Din acesta cauza, in exemplul dat mai sus o sa ne trezim ca elementul nu este gasit niciodata, chiar daca teoretic in colectie avem cheie data.
Pentru toate tipurile de date pe care o sa le folosim pe post de cheie, este nevoie sa facem override la GetHashCode si Equal. Mai jos putem sa gasim un exemplu pentru clasa DoorKey
class Doorkey
{
    int RoomNumber { get; set; }
    int DoorNumber {get; set; }
    public override bool Equals(object obj)
    {
        if( obj == null
            || !( obj is DoorKey ))
        {
            return false
        }
        return RoomNumber == obj.RoomNumber
            && DoorNumber == obj.DoorNumber
    }
    public override int GetHashCode()
{
    return RoomNumber.GetHashCode();
}
}
Pentru metoda GetGashCode, aceasta nu este cea mai buna implementara. Cea mai importanta regula pe care metoda GetHashCode trebuie sa o respecte este ca valoarea returnata sa fie aceiasi pentru un obiect care are aceiasi stare( valoare).
Asa ca, de fiecare data cand lucram cu dictionarele( maps) din .NET, iar cheile sunt reprezentate de tipuri custom, nu trebuie sa uitam sa facem ovveride la GetHashCode si Equals, in caz contrat o sa ne trezim ca nu putem sa obtinem datele pe baza cheii.

Comments

  1. Romanian language correction: se spune "cheile" :)

    ReplyDelete

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