What is a singleton in C#?

问题:

Pretty straight forward question.

What is a Singleton and when should I use it?

回答1:

A singleton is a class which only allows one instance of itself to be created - and gives simple, easy access to said instance. The singleton premise is a pattern across software development.

There is a C# implementation \"Implementing the Singleton Pattern in C#\" covering most of what you need to know - including some good advice regarding thread safety.

To be honest, It\'s very rare that you need to implement a singleton - in my opinion it should be one of those things you should be aware of, even if it\'s not used too often.

回答2:

You asked for C#. Trivial example:

public class Singleton
{
    private Singleton()
    {
        // Prevent outside instantiation
    }

    private static readonly Singleton _singleton = new Singleton();

    public static Singleton GetSingleton()
    {
        return _singleton;
    }
}

回答3:

What it is: A class for which there is just one, persistent instance across the lifetime of an application. See Singleton Pattern.

When you should use it: As little as possible. Only when you are absolutely certain that you need it. I\'m reluctant to say \"never\", but there is usually a better alternative, such as Dependency Injection or simply a static class.

回答4:

another way to implement singleton in c#, i personally prefer this way because you can access the instance of the singeton class as a property instead of a method.

public class Singleton
    {
        private static Singleton instance;

        private Singleton() { }

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

        //instance methods
    }

but well, as far as i know both ways are considered \'right\' so it\'s just a thing of personal flavor.

回答5:

A Singleton (and this isn\'t tied to C#, it\'s an OO design pattern) is when you want to allow only ONE instance of a class to be created throughout your application. Useages would typically include global resources, although I will say from personal experience, they\'re very often the source of great pain.

回答6:

using System;
using System.Collections.Generic;
class MainApp
{
    static void Main()
    {
        LoadBalancer oldbalancer = null;
        for (int i = 0; i < 15; i++)
        {
            LoadBalancer balancerNew = LoadBalancer.GetLoadBalancer();

            if (oldbalancer == balancerNew && oldbalancer != null)
            {
                Console.WriteLine(\"{0} SameInstance {1}\", oldbalancer.Server, balancerNew.Server);
            }
            oldbalancer = balancerNew;
        }
        Console.ReadKey();
    }
}

class LoadBalancer
{
    private static LoadBalancer _instance;
    private List<string> _servers = new List<string>();
    private Random _random = new Random();

    private static object syncLock = new object();

    private LoadBalancer()
    {
        _servers.Add(\"ServerI\");
        _servers.Add(\"ServerII\");
        _servers.Add(\"ServerIII\");
        _servers.Add(\"ServerIV\");
        _servers.Add(\"ServerV\");
    }

    public static LoadBalancer GetLoadBalancer()
    {
        if (_instance == null)
        {
            lock (syncLock)
            {
                if (_instance == null)
                {
                    _instance = new LoadBalancer();
                }
            }
        }

        return _instance;
    }

    public string Server
    {
        get
        {
            int r = _random.Next(_servers.Count);
            return _servers[r].ToString();
        }
    }
}

I took code from dofactory.com, nothing so fancy but I find this far good than examples with Foo and Bar additionally book from Judith Bishop on C# 3.0 Design Patterns has example about active application in mac dock.

If you look at code we are actually building new objects on for loop, so that creates new object but reuses instance as a result of which the oldbalancer and newbalancer has same instance, How? its due to static keyword used on function GetLoadBalancer(), despite of having different server value which is random list, static on GetLoadBalancer() belongs to the type itself rather than to a specific object.

Additionally there is double check locking here

if (_instance == null)
            {
                lock (syncLock)
                {
                    if (_instance == null)

since from MSDN

The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread tries to enter a locked code, it will wait, block, until the object is released.

so every-time mutual-exclusion lock is issued, even if it don\'t need to which is unnecessary so we have null check.

Hopefully it helps in clearing more.

And please comment if I my understanding is directing wrong ways.

回答7:

Whilst the there can only ever be one instance of a singleton, it is not the same as a static class. A static class can only contain static methods and can never be instantiated, whereas the instance of a singleton may be used in the same way as any other object.

回答8:

It\'s a design pattern and it\'s not specific to c#. More about it all over the internet and SO, like on this wikipedia article.

In software engineering, the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object. This is useful when exactly one object is needed to coordinate actions across the system. The concept is sometimes generalized to systems that operate more efficiently when only one object exists, or that restrict the instantiation to a certain number of objects (say, five). Some consider it an anti-pattern, judging that it is overused, introduces unnecessary limitations in situations where a sole instance of a class is not actually required, and introduces global state into an application.

You should use it if you want a class that can only be instanciated once.

回答9:

I use it for lookup data. Load once from DB.

public sealed class APILookup
    {
        private static readonly APILookup _instance = new APILookup();
        private Dictionary<string, int> _lookup;

        private APILookup()
        {
            try
            {
                _lookup = Utility.GetLookup();
            }
            catch { }
        }

        static APILookup()
        {            
        }

        public static APILookup Instance
        {
            get
            {
                return _instance;
            }
        }
        public Dictionary<string, int> GetLookup()
        {
            return _lookup;
        }

    }

回答10:

What is a singleton :
It is a class which only allows one instance of itself to be created, and usually gives simple access to that instance.

When should you use :
It depends on the situation.

Note : please do not use on db connection, for a detailed answer please refer to the answer of @Chad Grant

Here is a simple example of a Singleton:

public sealed class Singleton
{
    private static readonly Singleton instance = new Singleton();

    // Explicit static constructor to tell C# compiler
    // not to mark type as beforefieldinit
    static Singleton()
    {
    }

    private Singleton()
    {
    }

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

You can also use Lazy<T> to create your Singleton.

See here for a more detailed example using Lazy<T>

回答11:

Here\'s what singleton is: http://en.wikipedia.org/wiki/Singleton_pattern

I don\'t know C#, but it\'s actually the same thing in all languages, only implementation differs.

You should generally avoid singleton when it\'s possible, but in some situations it\'s very convenient.

Sorry for my English ;)

回答12:

Singleton class is used to create a single instance for whole of the application domain.

public class Singleton
{
    private static Singleton singletonInstance = CreateSingleton();

    private Singleton()
    {
    }

    private static Singleton CreateSingleton()
    {
        if (singletonInstance == null)
        {
            singletonInstance = new Singleton();
        }

        return singletonInstance;
    }

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

In this article it is described how we can create thread safe singleton class using readonly variable and their practical use in applications.

回答13:

Singleton is a type of remote object, which is used to service multiple clients. These objects maintain states rather than singel call object types