C# Singleton Pattern

Question

I use the singleton pattern in a lot of places, sometimes the constructor does nothing, other times it's initialising things. I wondered if there was a way to set up an abstract class to minimise my code repetition a bit, i.e. I don't need public static readonly Singleton _Instance = new Singleton(); in every single class, just one base class. I understand interfaces are not an option.

I've tried using the following (taken from here);

public abstract class Singleton<T> where T : new()
{
    static Singleton()
    {
    }

    private static readonly T _Instance = new T();

    public static T Instance
    {
        get { return _Instance; }
    }
}

The problem with this is that I can't override the constructor for the cases where I need to initialise things. Is what I'm trying to do even possible? Or should I just keep doing what I'm doing and not worry about a base singleton class?

I would recommend that you stop using singletons. They make software harder to test and can create dependencies across large parts of your code base. — Peter R
– Peter R, Commented Feb 18, 2014 at 8:10

Jon Skeet · Accepted Answer · 2014-02-18 08:13:10Z

8

I wondered if there was a way to set up an abstract class to minimise my code repetition a bit

No, there isn't. As soon as you've got an abstract class, you've got a class which can be instantiated multiple times. I've seen various people try to do something like this, but the end result is either not a singleton or is more complicated than just doing what you're already doing.

Is what I'm trying to do even possible? Or should I just keep doing what I'm doing and not worry about a base singleton class?

You shouldn't try to create a base singleton class - but if I were you I'd try to stop using quite so many singletons in the first place. The singleton pattern is very easily overused, and it's tantamount to an anti-pattern. See if you can refactor towards dependency injection, using a configuration which happens to create only one instance of each of these classes, but where that's a matter of configuration rather than enforced by the class itself.

Aside from anything else, unit testing involving singletons tends to be a pain, simply because it's global state which needs cleaning up between tests etc.

edited Feb 18, 2014 at 8:13

answered Feb 18, 2014 at 8:08

Jon Skeet

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5 Comments

Simon Over a year ago

I get the dependency injection argument, and I agree their use should be limited, but what if you need this class throughout your project? This means you require passing it to classes, then to child classes etc etc. Is this not a maintenance nightmare?

Jon Skeet Over a year ago

@Simon: I rarely find I need the same classes in a large proportion of the code, with the exception of logging - that's one of the very few places where I'm happy to stick with global state.

Simon Over a year ago

I too use it mainly for logging/error management, would the current state of the software count? I.e. If I need to store the current location in a location aware application, could this pattern fit/be suitable?

Jon Skeet Over a year ago

@Simon: No, I would usually have a LocationManager or something like that, and inject it only where needed. Easier to test, easier to see which parts really do depend on location etc.

Trent Over a year ago

John, the main reason I'm using the singleton pattern is to act as a cache. i.e. there's another class variable that maintains a list of database objects, so I don't have to call the database all the time. And the reason there's multiple singletons is I've split up the caching by database table, to limit the size of each class. Otherwise I'd have a massive class. Is this not a reasonable way of doing things?

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