Singleton Pattern in C#.

Singleton Pattern.cs

#region Singleton Pattern

// NOTE: The class is marked as sealed to prevent the inheritance of the class (i.e. use it as base/super class).
internal sealed class Singleton
{
    private static readonly object SingletonSyncRoot = new object();
    private static volatile Singleton _instanceSingleton;

    // NOTE: Default/Parameterless constructor is private to prevent instantiation of the class.
    private Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            // Double-checked locking pattern.
            if (_instanceSingleton != null)
            {
                return _instanceSingleton;
            }
            lock (SingletonSyncRoot)
            {
                return _instanceSingleton ?? (_instanceSingleton = new Singleton());
            }
        }
    }
}

// NOTE: The class is marked as sealed to prevent the inheritance of the class (i.e. use it as base/super class).
internal sealed class EagerSingleton
{
    // NOTE: Default/Parameterless constructor is private to prevent instantiation of the class.
    private EagerSingleton()
    {
    }

    // CLR eagerly initializes static member when the class is first used
    // CLR guarantees thread-safety for static initialization
    public static EagerSingleton Instance
    {
        get;
    } = new EagerSingleton();
}

#endregion Singleton Pattern