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How to extract a file extension from a string path

Cautand ceva printr-o aplicatie am gasit urmatoarele linii de cod:
foreach (Foo foo in contaner.Foos)
{
if ( foo .FileName.EndsWith(".exe") )
{
// executa o actiune
}
}
Ce vi se pare ciudat in codul de mai sus?
Ce mi-a sarit mie in ochi in primul moment a fost modul in care se verifica daca extensia la fisier este ".exe". .NET ne pune la dispoztie clasa System.IO.Path, prin intermediul careia putem sa extragem numele de fisier, extensia, full path-ul, root path etc.
In cazul nostru putem sa folosim metoda Path.GetExtension(string). Aceasta metoda o sa ne returneze extensia fisierului, daca fisierul are extensie, iar in cazul in care fisierul nostru nu are extensie atunci o sa returneze stringul gol( String.Empty).
Dar ce se intampla in cazul in care path-ul pe care noi il dam este NULL? Valoarea care o sa fie returnata o sa fie la randul ei NULL.
Codul de mai sus l-am putea rescrie in felul urmator:
foreach (Foo foo in contaner.Foos)
{
if ( Path.GetExtension(foo .FileName) == ".exe" )
{
// executa o actiune
}
}
Nu trebuie sa uitam ca exista si metoda Path.HasExtension(string), care o sa returneze TRUE, daca path-ul dat contine o extensie. In cazul de mai sus nu e nevoie sa facem aceasta verificare, deoarece GetExtension o sa ne returneze mereu string-ul Empty daca fisierul nu contine nici o extensie.
Dar trebuie sa avem grija sub ce forma este extensia. Daca pentru noi "exe" este egal cu "eXe" si cu "EXE", atunci verificarea de egalitate trebuie sa fie facuta folosind metoda Equals. Codul nostru ar trebuie sa fie rescris in felul urmator:
foreach (Foo foo in contaner.Foos)
{
if ( Path.GetExtension(foo .FileName).Equals(".exe", StringComparison.InvariantCultureIgnoreCase )
{
// executa o actiune
}
}
As vrea sa atrag atentia ca metoda Path.GetExtension(string) ne returneaza extensia unui fisier inclusiv cu ".". De exemplu pentru "foo.exe", valoarea returnata o sa fie ".exe".
Enjoy!

Comments

  1. Daca nu vrei sa afli care e extensia (fara sa o stii in prealabil), ci doar vrei sa verifici ca extensia e sau nu un anume string (".exe" in cazul asta), prima varianta mi se pare ok
    (si poate si o idee mai eficienta daca se foloseste OrdinalIgnoreCase, nu InvariantCultureIgnoreCase).

    Singura lipsa ar fi faptul ca e case si culture-sensitive, dar EndsWith are si un overload care accepta un StringComparation.

    Normal, prima varianta e ok doar daca presupunem ca definitia a ce inseamna 'extension' ramane aceeasi in viitor.. :)

    ReplyDelete
  2. .ToLower poate sa fie si aceasta o solutie. Personal prefer sa folosesc Equal in loc de ToLower si ==. Mi se pare mult mai clar si nu e nevoie sa apelez ToLower pentru ambele string-uri pe care le compar( daca nu sunt constante). Dar depinde de caz si de preferintele fiecaruia.

    Pentru a extrage extensia unui fisier nu as evita EndsWith. Daca intr-un API a unui framework avem deja un mecanism pentru o anumite functionalitate deja implementat, prefer sa o folosesc pe aceasta( teoretic ar trebuii sa fie mai sigura si sa nu genereze erori).
    In acest caz mi se pare mult mai lizibil codul folosind GetExtension. Cel care citeste isi da seama mai usor ce se intampla acolo.

    ReplyDelete
  3. Pai asta ziceam, ca in cazul de mai sus nu e nevoie sa extragi extensia unui file name (pentru ca stii deja care ar trebui sa fie extensia).. :)
    Intr-adevar, cu GetExtension codul e ceva mai usor de inteles - o alta varianta, ca sa fie clara intentia ar fi:
    string expectedExtension = ".exe";
    if ( foo.FileName.EndsWith(expectedExtension, ...) ) ...

    ReplyDelete
  4. Hai ca facem semantica cu totii...

    ReplyDelete

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