Singleton

The Singleton design pattern seems generally frowned upon, except for use in logging classes.  Below are several variants of the Singleton design pattern, of which, only the thread-safe version merits attention:


Salient Chacteristic(s)

• A private constructor
• Static variable for self-tracking

Three (3) variants

• Eager

• Lazy

• Thread-safe (Double-checked Locking)

Code


namespace DesignPatterns
{

    //double-checked locking singleton
    //required for threaded environments
    public class DoubleCheckingSingleton
    {
        private static volatile DoubleCheckingSingleton _Instance = null;
        private static object _SyncRoot = new object();
    
        //Private constructor prevents instantiation from other classes
        private DoubleCheckingSingleton()
        {
            //class contruction
        }
        
        public static DoubleCheckingSingleton Instance
        {
            get
            {
                if (_Instance == null)
                {
                    lock (_SyncRoot)
                    {
                        if (_Instance == null)
                        {
                            _Instance = new DoubleCheckingSingleton();
                        }
                    }
                }
                return _Instance;
            }   
        }
    }

    //lazy initialization of singleton
    public class LazySingleton 
    {
        private static LazySingleton _Instance = null;
    
        // Private constructor prevents instantiation from other classes
        private LazySingleton() 
        {
            //class contruction
        }


        public LazySingleton getInstance() 
        {
            if (_Instance == null)
            {
                _Instance = new LazySingleton();
            }
            else
            {
                return _Instance;
            }
            return _Instance;
        }
    }


    //eager initialization of singleton
    public class EagerSingleton
    {
        private static EagerSingleton _Instance = new EagerSingleton();

        // Private constructor prevents instantiation from other classes
        private EagerSingleton()
        {
            //class contruction
        }

        public EagerSingleton getInstance()
        {
            return _Instance;
        }
    }
}