C# - Singleton Pattern

Question

As you can see from my nickname I'm newbie actually learning about Singleton pattern, where I got one problem. Before I've learned that static constructors are always executed before the standard constructors, but in this code below, the result is different, first I see the "Insta" string then the "Static", why does it happen ?

sealed class Singleton
{
    private static readonly Singleton instance;

    private Singleton()
    {
        Console.WriteLine("Insta");
    }

    static Singleton()
    {
        instance  = new Singleton();
        Console.WriteLine("Static");
    }

    public static Singleton Instance
    {
        get
        {
            return instance;
        }
    }
}

class Program
{
    static void Main()
    {
        Singleton s1 = Singleton.Instance;

    }

}

Not sure on reasoning of your expectation - "static constructor called first" does not mean "all code in static constructor magically executed before all calls to class members" (instance constructor in your case). — Alexei Levenkov
– Alexei Levenkov, Commented Jul 3, 2013 at 20:50

Tigran · Accepted Answer · 2013-07-03 20:45:20Z

13

If you will write

static Singleton()
{
    Console.WriteLine("Static"); //THIS COMES FIRST
    instance  = new Singleton();

}

you would see what you expect

static ctor is executed at first, as expected, but you print on console after the instance = new Singleton(); line, but this line execute instance ctor, so "inst".

So execution flow:

static ctor
- instance = new Singleton();
  - instance ctor prints "insta"
- "static"

answered Jul 3, 2013 at 20:45

Tigran

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4 Comments

CSharpBeginner Over a year ago

Right my fault, thanks. But I have one more ask for you what if i write "private static readonly Singleton instance = new Singleton();" Will compiler move this expression to static ctor or what?

konkked Over a year ago

@CSharpBeginner no, the Singleton instance will be constructed the first time the class is called, you would no longer need a static constructor

CSharpBeginner Over a year ago

So this initializtion works the same way static ctor does right ?

Tigran Over a year ago

@CSharpBeginner: if saying "same way", you mean initialized only once, yes.

Community · Accepted Answer · 2020-06-20 09:12:55Z

9

See the MSDN pattern here for a quality explanation of the singleton pattern.

MSDN recommends you should write it as below so it is thread safe:

using System;

public sealed class Singleton
{
   private static volatile Singleton instance;
   private static object syncRoot = new Object();

   private Singleton() {}

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

         return instance;
      }
   }
}

By the way, this pattern has the following advantage over static constructor:

The instantiation is not performed until an object asks for an instance; this approach is referred to as lazy instantiation. Lazy instantiation avoids instantiating unnecessary singletons when the application starts.

See if this feets your need, and if so, implement this solution.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 3, 2013 at 20:48

saamorim

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2 Comments

wjmolinam Over a year ago

What does thread safety have to do with a conceptual question like this one?

saamorim Over a year ago

My point is to present the MSDN article in how to implement a singleton solution. No need to reinvent the wheel. Since he is a beginner it is always good to have a good solid article to read and understand a specific pattern.

Iucounu · Accepted Answer · 2013-07-03 20:53:07Z

1

The static method is invoked first. Here's the proof--change your code to the following:

static Singleton()
    {
        Console.WriteLine("Static method start");
        instance  = new Singleton();
        Console.WriteLine("Static method end");
    }

answered Jul 3, 2013 at 20:53

Iucounu

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