Abstract base class to force each derived classes to be Singleton

How do I make 开发者_如何学Goan abstract class that shall force each derived classes to be Singleton ? I use C#.

---

that would not work because the singleton needs somwhere a static access and that can not be forced.

for singletonimplemention + examples see: Implementing the Singleton Pattern in C#

---

When you want to enfore compile time checking, this is not possible. With runtime checking you can do this. It's not pretty, but it's possible. Here's an example:

public abstract class Singleton
{
    private static readonly object locker = new object();
    private static HashSet<object> registeredTypes = new HashSet<object>();

    protected Singleton()
    {
        lock (locker)
        {
            if (registeredTypes.Contains(this.GetType()))
            {
                throw new InvalidOperationException(
                    "Only one instance can ever  be registered.");
            }
            registeredTypes.Add(this.GetType());
        }
    }
}

public class Repository : Singleton
{
    public static readonly Repository Instance = new Repository();

    private Repository()
    {
    }
}

---

Singleton means having private constructors. But you know private members cannot be inherited. In C++ there were templates, so you could create a singleton from a template class. in C#, there aren't templates, so you have to write your own private constructors for every singleton you want.

---

Classes in java or C# are not "first-class". The static part of a class can not be inherited or overridden by subclasses. See this answer for more details. Plus, you don't have a concept of meta-class.

In languages like Smalltalk or Ruby, you can define a new metaclass Singleton which defines a method getInstance. Then you can define ClassA and ClassB be instances of the Singleton metaclass. Then both classes automatically expose a method getInstance which can be used to create instances objectA or objectB. Isn't that cool? Well, in practice you don't use metaclass frequently, and the singleton is actually the only usage of them that makes sense and that I'm aware of.

---

Here's an (ugly) way to do this. It could probably be simplified and improved, but it's my first go at it.

The idea is to first make the base class a generic abstract class (as mentioned in comments above), but the type parameter is constrained to derive from the base class itself. This allows the base class to handle the singleton instance of the derived type. Note all derived classes should be sealed, as with any singleton class.

Next, a protected constructor is allowed, but is required to accept an instance of a special class, SingletonKey, which is a modified singleton. Derived classes have access to the SingletonKey class definition, but the base class retains private control over the only allowed instance, and thus over construction of all derived objects.

Third, the base class will need to be able to call the constructor of the derived class, but this is slightly tricky. The compiler will complain if you try to call the derived keyed constructor, since it isn't guaranteed to exist. The solution is to add a static delegate that the derived class must initialize. So any derived classes will need to provide a simple initialization method. This initialization method should be called explicitly before attempting to access the instance for the first time in code, or a runtime error will result.

public abstract class Singleton<T> where T : Singleton<T>
{
    protected Singleton(SingletonKey key) { }
    private static SingletonKey _key;
    private static SingletonKey Key
    {
        get
        {
            if (_key == null) SingletonKey.Initialize();
            return _key;
        }
    }
    protected class SingletonKey
    {
        private SingletonKey()
        {
        }
        public static void Initialize()
        {
            if (_key == null)
            {
                _key = new SingletonKey();
            }
        }
    }

    protected static Func<SingletonKey, T> Creator;
    private static T instance;
    public static T Instance
    {
        get
        {
            if (instance == null) instance = Creator(Key);
            return instance;
        }
    }
}

public class MySingleton : Singleton<MySingleton>
{
    public string Name { get; set; }
    public static void Initialize()
    {
        Creator = (key) => new MySingleton(key);
    }
    protected MySingleton(SingletonKey key) : base(key)
    {
    }
}

---

I believe I tried to achieve something similar i.e. enforce a common interface and the singleton pattern on a group of classes. This was my solution:

// Common interface of my singleton classes
public interface IMySingletonClass
{
    string ValueGetter();

    void ValueSetter(string value);
}

// Generic abstract base class
public abstract class Singleton<T>: IMySingletonClass
{
    private static readonly object instanceLock = new object();
    private static T instance; // Derived class instance

    // Protected constructor accessible from derived class
    protected Singleton()
    {
    }

    // Returns the singleton instance of the derived class
    public static T GetInstance()
    {
        lock (instanceLock)
        {
            if (instance == null)
            {
                instance = (T)Activator.CreateInstance(typeof(T), true);
            }
            return instance;
        }
    }

    // IMySingletonClass interface methods
    public abstract string ValueGetter();

    public abstract void ValueSetter(string value);
}

// Actual singleton class
public class MySingletonClass : Singleton<MySingletonClass>
{
    private string myString;

    private MySingletonClass()
    {
        myString = "Initial";
    }

    public override string ValueGetter()
    {
        return myString;
    }

    public override void ValueSetter(string value)
    {
        myString = value;
    }
}

Here is a simple test:

class Program
{
    static void Main(string[] args)
    {
        MySingletonClass r1 = MySingletonClass.GetInstance();
        Console.WriteLine("R1 value = {0}", r1.ValueGetter());
        r1.ValueSetter("Changed through R1");
        MySingletonClass r2 = MySingletonClass.GetInstance();
        Console.WriteLine("R2 value = {0}", r2.ValueGetter());

        Console.ReadKey();
    }
}

Please note that you can easily remove the common interface from the generic abstract singleton class if you just need the basic "template".

---

Here my implementation of Singleton inheritance:

using System;
using System.Reflection;

namespace Mik.Singleton
{
    class Program
    {
        static void Main()
        {
            //You can not create an instance of class directly
            //Singleton1 singleton1 = new Singleton1();

            Singleton1 singleton1 = Singleton1.Instance;
            Singleton2 singleton2 = Singleton2.Instance;

            Console.WriteLine(singleton1.Singleton1Text);
            Console.WriteLine(singleton2.Singleton2Text);

            Console.ReadLine();
        }
    }

    public class SingletonBase<T> where T : class
    {
        #region Singleton implementation

        private static readonly object lockObj = new object();
        private static T _instance;

        protected SingletonBase() { }

        public static T Instance
        {
            get
            {
                if (_instance == null)
                {
                    lock (lockObj)
                    {
                        if (_instance == null)
                            _instance = CreateInstance();
                    }
                }
                return _instance;
            }
        }

        private static T CreateInstance()
        {
            ConstructorInfo constructor = typeof(T).GetConstructor(
                            BindingFlags.Instance | BindingFlags.NonPublic,
                            null, new Type[0],
                            new ParameterModifier[0]);

            if (constructor == null)
                throw new Exception(
                    $"Target type is missing private or protected no-args constructor: {typeof(T).FullName}");
            try
            {
                T instance = constructor.Invoke(new object[0]) as T;

                return instance;
            }
            catch (Exception e)
            {
                throw new Exception(
                    "Failed to create target: type=" + typeof(T).FullName, e);
            }
        }

        #endregion Singleton implementation
    }

    public class Singleton1 : SingletonBase<Singleton1>
    {
        private Singleton1() { }

        public string Singleton1Text { get; } = "Singleton1Text value";
    }

    public class Singleton2 : SingletonBase<Singleton2>
    {
        private Singleton2() { }

        public string Singleton2Text { get; } = "Singleton2Text value";
    }
}