Singleton or static - what should I use?

Question

I don't know what should I use. I have 3 classes. PasswordService, SettingsService, FileService. These classes each contain about 2 methods. The methods are being used in more assemblies. Now I'm using it as a singleton. But I'm not sure if I should. I think a static class would be enough. What do you think?

CODE:

public class PasswordService
{

    private PasswordService(){}

    private static PasswordService _instance;

    public static PasswordService Instance
    {
        get { return _instance ?? (_instance = new PasswordService()); }
    }

    public byte[] EncryptPassword(string password)
    {

        var protectedPass = Encoding.UTF8.GetBytes(password);
        return ProtectedData.Protect(protectedPass, null);
    }

    public string DecryptPassword(byte[] encryptedPassword)
    {
        var unprotectedPass = ProtectedData.Unprotect(encryptedPassword, null);
        return Encoding.UTF8.GetString(unprotectedPass, 0, unprotectedPass.Length);
    }
}

possible duplicate of [Should I use a singleton?](http://stackoverflow.com/questions/1676296/should-i-use-a-singleton) — H H, Mar 18 '13 at 11:16
I would say that a singleton is almost always evil. Use an instance of each of these services. You will end up with a more flexible design where you can see your real dependences. — Ignacio Soler Garcia, Mar 18 '13 at 11:16
@SoMoS: You're essentially saying "Singletons are evil, so use them". Or did you want to say "Create a new instance everywhere you want to use those services"? — Nuffin, Mar 18 '13 at 11:19
@Nuffin: sorry, of course I meant "create a new instance everywhere you need one". If several objects have to share the same instance pass it as a parameter on the constructor or in the way that best fits the design. — Ignacio Soler Garcia, Mar 18 '13 at 11:21

score 2 · Accepted Answer · answered Mar 18 '13 at 11:22

2

You don't have any state in your class, so I do not see any reason to use instance of class. I recommend you to use static classes.

answered Mar 18 '13 at 11:22

Mikhail Gubanov

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No one of these classes keeps any state. Only methods are in. – JuP Mar 18 '13 at 11:23

score 1 · Answer 2 · answered Mar 18 '13 at 11:17

1

1 - It is right to create a singleton for these services since these are seems to be handle one specific task.

2 - Avoid Static as much as possible as you can not mock it efficiently if you use TDD and do automatic unit testing with contineous Integration server.

answered Mar 18 '13 at 11:17

TalentTuner

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So It can be a singleton or should I re-create it to static? – JuP Mar 18 '13 at 11:19

Matthias Meid · Answer 3 · 2013-03-18T11:26:15.287

1

I recommend you to keep the Singleton, unless you're using instances or DI. A Singleton can be refactored into instances quite easily, whereas static classes must be re-implemented to being non-static. Moreover, you can replace instances with test dummies, whereas replacing a static implementation is hardly possible.

For instance you might come across a situation where your programm must handle multiple FileConfiguration instances, for two distinct config files. A Singleton can be split into a two-instance pool.

I came across this issue with a DAO class which used to be static and capable of connecting to one DB. We had to refactor it, since a new requirement included supporting n>1 databases in one program instance.

As Mikhail points out, use static only for truly stateless stuff. A config file or a chosen password hashing algorithm in a static field is already a state, as well as the connection string in my example above is - even though they might never change at runtime.

edited Mar 18 '13 at 11:26

answered Mar 18 '13 at 11:20

Matthias Meid

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But I don't think, that I keep any state. The methods only return the result. The classes haven't any fields. – JuP Mar 18 '13 at 11:27
1

@JurajP As for the `PasswordService` you're right, there's technically no state. However, for instance `EncryptPassword` and `DecryptPassword` are related to each other, as they share the use of `Encoding.UTF8`. Here and in other services of yours this might become a state like `readonly Encoding enc = Encoding.UTF8` for easier and less error-prone refactoring. I admit this sounds pretty far-fetched, but I regretted some static methods I made in the past that weren't *stateless and technically unrelated to each other*. Hope I expressed my point well enough... :) – Matthias Meid Mar 18 '13 at 11:37

Singleton or static - what should I use?

3 Answers3