Avoiding static variables with Singleton

Question

A colleague of mine told me that I should never use static variables because if you change them in one place, they are changed everywhere. He told me that instead of using static variables I should use Singleton. I know that Singleton is for limitation of the number of instances of one class to one. How can Singleton help me with static variables?

Answer 1

Let's address your statements one at a time:

A colleague of mine told me that I should never use static variables because if you change them in one place, they are changed everywhere.

It seems fairly clear that your colleague means the basic feature of static variables: there is only one instance of a static variable. No matter how many instances of any class you create, any access to a static variable is to the same variable. There is not a separate variable for each instance.

He told me that instead of using static variables I should use Singleton.

This is not good global advice. Static variables and singletons aren't in competition with each other and aren't really substitutes for each other. A singleton is an instance of a class, managed in such a way that only one instance is possible to create. A static variable is similarly tied to exactly one (static) instance of a class, but could be assigned with not only a class instance but any data type such as a scalar. In actuality, to effectively use the singleton pattern, you must store it in a static variable. There is no way to "use a singleton instead of a static variable".

On the other hand, perhaps he meant something slightly different: perhaps he was trying to say that instead of your static class having many different static variables, methods, properties, and fields (altogether, members) that function as if they were a class, you should make those fields non-static, and then expose the wrapping class as a Singleton instance. You would still need a private static field with a method or property (or perhaps just use a get-only property) to expose the singleton.

I know that Singleton is for limitation of the number of instances of one class to one. How can Singleton help me with static variables?

A static class's variables and a singleton are alike in that they both are instantiated once (the former enforced by the compiler and the latter enforced by your implementation). The reason you'd want to use a singleton instead of a static variable inside of a class is when your singleton needs to be a true instance of a class, and not consist simply of the collected static members of a static class. This singleton then gets assigned to a static variable so that all callers can acquire a copy of that same instance. As I said above, you can convert all the different static members of your static class to be instance members of your new non-static class which you will expose as a singleton.

I would also like to mention that the other answers given so far all have issues around thread safety. Below are some correct patterns for managing Singletons.

Below, you can see that an instance of the Singleton class, which has instance (or non-static) members, is created either by static initialization or within the static constructor, and is assigned to the variable _singleton.. We use this pattern to ensure that it is instantiated only once. Then, the static method Instance provides read-only access to the backing field variable, which contains our one, and only one, instance of Singleton.

public class Singleton {
   // static members
   private static readonly Singleton _singleton = new Singleton();
   public static Singleton Instance => _singleton

   // instance members
   private Singleton() { } // private so no one else can accidentally create an instance
   public string Gorp { get; set; }
}

or, the exact same thing but with an explicit static constructor:

public class Singleton {
   // static members
   private static readonly Singleton _singleton; // instead of here, you can...
   static Singleton() {
      _singleton = new Singleton(); // do it here
   }
   public static Singleton Instance => _singleton;

   // instance members
   private Singleton() { } // private so no one else can accidentally create an instance
   public string Gorp { get; set; }
}

You could also use a property default without an explicit backing field (below) or in a static constructor can assign the get-only property (not shown).

public class Singleton {
   // static members
   public static Singleton Instance { get; } = new Singleton();

   // instance members
   private Singleton() { } // private so no one else can accidentally create an instance
   public string Gorp { get; set; }
}

Since static constructors are guaranteed to run exactly once, whether implicit or explicit, then there are no thread safety issues. Note that any access to the Singleton class can trigger static initialization, even reflection-type access.

You can think of static members of a class as almost like a separate, though conjoined, class:

1. Instance (non-static) members function like a normal class. They don't live until you perform new Class() on them. Each time you do new, you get a new instance. Instance members have access to all static members, including private members (in the same class).

2. Static members are like members of a separate, special instance of the class that you cannot explicitly create using new. Inside this class, only static members can be accessed or set. There is an implicit or explicit static constructor which .Net runs at the time of first access (just like the class instance, only you don't explicitly create it, it's created when needed). Static members of a class can be accessed by any other class at any time, in or out of an instance, though respecting access modifiers such as internal or private.

Answer 2

EDIT @ErikE's response is the correct approach.

For thread safety, the field should be initialized thusly:

private static readonly Singleton instance = new Singleton();

---

One way to use a singleton (lifted from http://msdn.microsoft.com/en-us/library/ff650316.aspx)

using System;

public class Singleton
{
   private static Singleton instance;

   private Singleton() {}

   public static Singleton Instance
   {
      get 
      {
         if (instance == null)
         {
            instance = new Singleton();
         }
         return instance;
      }
   }

   /// non-static members
   public string Foo { get; set; }        
   
}

Then,

var foo = Singleton.Instance.Foo;
Singleton.Instance.Foo = "Potential thread collision here.";

Note that the instance member is a static field. You can't implement a singleton without using a static variable, and (I seem to recall - it's been awhile) this instance will be shared across all requests. Because of that, it's inherently not thread safe.

Instead, consider putting these values in a database or other persistent store that's more thread-friendly, and creating a class that interfaces with that portion of your database to provide transparent access.

public static class Foo
{
  public static string Bar
  {
    get { /// retrieve Bar from the db }
    set { /// update Bar in the db }
  }
}