Mixing Dependency Injection with the Singleton Design Pattern - c#

I have a Singleton in my Project looking like this:
//Create a Singleton
static MySingleton Instance;
private static readonly object Padlock = new object();
/// <summary>
/// Singelton Method, to make sure only one instance of this class exists at runtime.
/// </summary>
/// <returns></returns>
public static MySingleton GetInstance()
{
//Thread Safety
lock (Padlock)
{
if (Instance == null)
{
Instance = new MySingleton();
}
return Instance;
}
}
private MySingleton()
{
}
//[...]
The Singleton contains several other classes as Properties, which should never be null.
Normally I'd use DependencyInjection to gurantee each new object gets all necessary parameters on instantiation.
Like this:
IHelperClass Helper {get; set;}
IExectiveClass Executive {get; set;}
public NotMySingleton(IHelperclass helper, IExecutiveClass executive)
{
Helper = helper;
Executive = executive;
}
But I have no idea how to combine DependencyInjection with a Singleton Pattern.
Is there a way to use Singletons with DependenyInjection? What is common practice? What alternatives do I have (if this is not a suitable option)?


All DI frameworks provide a way to define the lifetime of objects instantiated by the DI container.
One lifetime scope is usually Singleton.
If you let the DI framework control the lifetime of your MySingleton, DI will just work.
One remark though, any class which gets injected into the singleton will also be a singleton, no matter how the scope of those dependencies was defined.
If you cannot use or don't want to use the DI framework to create your MySingleton instance, one way to achieve the same would be to make the DI container available and resolve whatever the instance needs from the DI container, manually.
Some might consider this an antipattern though.


Using a DI framework that provides a DI container would be the way to go, however for cases where this is not possible, perhaps one can use the GetInstance() method to inject the dependencies.
Something like this.
//Create a Singleton
static MySingleton Instance;
private static readonly object Padlock = new object();
//The method that would be used by production code to use the class.
public static MySingleton GetInstance()
{
if (Instance != null)
{
return Instance;
}
IDependentService srv = new ConcreteService();
return GetInstance(srv);
}
//The method that allows for injecting dependencies for unit testing.
public static MySingleton GetInstance(IDependentService service)
{
if (Instance == null)
{
lock (Padlock)
{
if (Instance == null)
{
Instance = new MySingleton(service);
}
}
return Instance;
}
private MySingleton(IDependentService service)
{
//...
}

Related

ASP.NET Core - use configurations in singleton

How can I use IConfiguration to initialize singleton?
public class Singleton
{
Singleton(string apiKey)
{
this.apiKey = apiKey;
}
private static readonly object instanceGeneratorLock = new object();
private static Singleton instance = null;
private readonly string apiKey;
public static Singleton Instance
{
get
{
if (instance == null)
{
lock (instanceGeneratorLock)
{
if (instance == null)
{
/* use IConfiguration here */
var apiKey = System.Configuration.ConfigurationManager.AppSettings["ApiKey"];
instance = new Singleton(apiKey);
}
}
}
return instance;
}
}
}

Instead of manually creating the singleton and using it like regularly, I'd recommend you use a dependency injected singleton. This is very easy to achieve and can be done like so, inside your ConfigureServices method of Startup.cs:
services.AddSingleton(typeof(Singleton));
This will register your Singleton which will from then on forwards be able to be injected into any other class constructed via dependency injection (DI).
There are multiple overloads for this, for different use cases, all documented here

Do two instances of a singleton class have the same property value?

I am new to design patterns in C#. Can anyone please give me some instructions about the implementation of a Singleton class. I just implemented a tutorial but I am not able to understand use of singleton class with this "singleton means we can create only one instance of a class". Then why we don't access property which is written in the singleton class using two different instance of the class.
Please look at my code and give me instructions about the mistake I made.
static void Main(string[] args)
{
Singleton instance = Singleton.getInstance();
instance.Message = "Text Message";
Singleton instance1 = Singleton.getInstance();
Console.WriteLine(instance.Message);
Console.WriteLine(instance1.Message);
Console.ReadKey();
}
class Singleton
{
private static Singleton singleton=null;
private Singleton(){}
public static Singleton getInstance()
{
if (singleton!=null)
{
return singleton;
}
return new Singleton();
}
public string Message{get; set;}
}

Your singleton is incorrect.
More correct version:
class Singleton
{
private static Singleton singleton = null;
private Singleton(){}
public static Singleton getInstance()
{
if (singleton!=null)
{
return singleton;
}
return (singleton = new Singleton()); //here is
}
public string Message{get; set;}
}
And very good solution:
class Singleton
{
private static Lazy<Singleton> singleton = new Lazy<Singleton>(()=> new Singleton());
private Singleton() { }
public static Singleton getInstance()
{
return singleton.Value;
}
public string Message { get; set; }
}
It has no problems with thread-safity and lazy initialization.
By default, all public and protected members of the Lazy class are
thread safe and may be used concurrently from multiple threads. These
thread-safety guarantees may be removed optionally and per instance,
using parameters to the type's constructors.

Your Singleton implementation is incorrect.
A Singleton is designed to only allow none or a single instance at all times.
This is where you went wrong:
public static Singleton getInstance()
{
// "singleton" will always be null.
if (singleton != null)
{
return singleton;
}
// Always returns new instance rather than existing one.
return new Singleton();
}
To fix it you should write:
public static Singleton getInstance()
{
// Return the instance we might have stored earlier.
if (singleton != null)
return singleton;
// Now we store the only instance that will ever be created.
singleton = new Singleton();
return singleton;
}
Note that this is not thread safe if called multiple times in parallel.
As a resource I can recommend Jon Skeet's post:
http://csharpindepth.com/Articles/General/Singleton.aspx
He explaines six different solutions (including thread-safety and the Lazy(T)) to the Singleton Pattern and how to code them.

Does two instance of singleton class has a same property value?
The answer to your question is yes, they has the same property value.
Important thing is understand why, and the reason is the core of what a singleton is. So, why?:
Because you are confusing two references with two instance.
There are no two instances, there are always one none or one instance of the singleton class.
In your code, singleton variable and singleton1 variable are pointing both to the same object, the singleton, and the reason is because of the implementation of the method getInstance(), is simple to understand :
If method is called for the very first time, then it creates for unique time the singleton object with method new.
If method is called after a first time, it will return the singleton object created in the first call of method.
So, no matter how many variables of type Singleton you have, you will always have only one Singleton created with the new method, the instance, the singleton instance.

Why don't pre-create instance before return in Singleton pattern

I've seen many people write singleton like this
public class Singleton
{
private static Singleton _instance = null;
public static Singleton Instance
{
get
{
if (_instance == null)
{
_instance = new Singleton();
}
return _instance;
}
}
}
What is the difference from this code
public class Singleton
{
private static Singleton _instance = new Singleton();
public static Singleton Instance
{
get { return _instance; }
}
}
Nowaday(C# 6), we have Getter-only auto-properties, is this difference from above(I prefer to write it like this)
public class Singleton
{
public static Singleton Instance { get; } = new Singleton();
}
From what I've known, static field is guaranteed to be ready before I access it for the first time, so it's nothing different, only thing that different is in the first case, I will know when instance is created.
Is there anything more than this or I misunderstand everything?

It is called "Lazy" which postpones the creation of value to first request.
Create the object at the very beginning.
Simplified version of 2.
Or, you can simply use "Lazy" class which many people neglect.
public class Singleton
{
private static Lazy<Singleton> instance = new Lazy<Singleton>();
public static Singleton Instance => instance.Value;
}
"Lazy" is good for large programs.
Reduce startup time since creation is postponed.
Save resource if eventually class is not used.

HttpContext like Object

I like how HTTPContext.Current works. Is there any way that I can implement a similar object that has no relations to HTTPContextBase? Basically, I would like to create a UserContext. Then in the DAL, I could simple query this UserContext for user-specific information. This object would have to be thread-safe and work in both an ASP.NET environment (so THREAD STATIC attribute won't work) and console/library environments.

HttpContext.Current is a Singleton. Thread safe implementation is like this:
using System;
public sealed class Singleton
{
private static volatile Singleton instance;
private static object syncRoot = new Object();
private Singleton() {}
public static Singleton Current
{
get
{
if (instance == null)
{
lock (syncRoot)
{
if (instance == null)
instance = new Singleton();
}
}
return instance;
}
}
}
However using Singleton pattern is not good idea. It is almost "anti-pattern". This obstructs unit testing. Instead of this better to use Dependency Injection Container. http://en.wikipedia.org/wiki/Dependency_injection

How to implement a singleton in C#?

How do I implement the singleton pattern in C#? I want to put my constants and some basic functions in it as I use those everywhere in my project. I want to have them 'Global' and not need to manually bind them every object I create.

If you are just storing some global values and have some methods that don't need state, you don't need singleton. Just make the class and its properties/methods static.
public static class GlobalSomething
{
public static int NumberOfSomething { get; set; }
public static string MangleString( string someValue )
{
}
}
Singleton is most useful when you have a normal class with state, but you only want one of them. The links that others have provided should be useful in exploring the Singleton pattern.

Singletons only make sense if both of these conditions are true:
The object must be global
There must only ever exist a single instance of the object
Note that #2 does not mean that you'd like the object to only have a single instance - if thats the case, simply instantiate it only once - it means that there must (as in, it's dangerous for this not to be true) only ever be a single instance.
If you want global, just make a global instance of some (non signleton) object (or make it static or whatever).
If you want only one instance, again, static is your friend. Also, simply instantiate only one object.
Thats my opinion anyway.

Singleton != Global. You seem to be looking for the keyword static.

You can really simplify a singleton implementation, this is what I use:
internal FooService() { }
static FooService() { }
private static readonly FooService _instance = new FooService();
public static FooService Instance
{
get { return _instance; }
}

Hmm, this all seems a bit complex.
Why do you need a dependency injection framework to get a singleton? Using an IOC container is fine for some enterprise app (as long as it's not overused, of course), but, ah, the fella just wants to know about implementing the pattern.
Why not always eagerly instantiate, then provide a method that returns the static, most of the code written above then goes away. Follow the old C2 adage - DoTheSimplestThingThatCouldPossiblyWork...

I would recommend you read the article Exploring the Singleton Design Pattern available on MSDN. It details the features of the framework which make the pattern simple to implement.
As an aside, I'd check out the related reading on SO regarding Singletons.

Ignoring the issue of whether or not you should be using the Singleton pattern, which has been discussed elsewhere, I would implement a singleton like this:
/// <summary>
/// Thread-safe singleton implementation
/// </summary>
public sealed class MySingleton {
private static volatile MySingleton instance = null;
private static object syncRoot = new object();
/// <summary>
/// The instance of the singleton
/// safe for multithreading
/// </summary>
public static MySingleton Instance {
get {
// only create a new instance if one doesn't already exist.
if (instance == null) {
// use this lock to ensure that only one thread can access
// this block of code at once.
lock (syncRoot) {
if (instance == null) {
instance = new MySingleton();
}
}
}
// return instance where it was just created or already existed.
return instance;
}
}
/// <summary>
/// This constructor must be kept private
/// only access the singleton through the static Instance property
/// </summary>
private MySingleton() {
}
}

Static singleton is pretty much an anti pattern if you want a loosely coupled design. Avoid if possible, and unless this is a very simple system I would recommend having a look at one of the many dependency injection frameworks available, such as http://ninject.org/ or http://code.google.com/p/autofac/.
To register / consume a type configured as a singleton in autofac you would do something like the following:
var builder = new ContainerBuilder()
builder.Register(typeof(Dependency)).SingletonScoped()
builder.Register(c => new RequiresDependency(c.Resolve<Dependency>()))
var container = builder.Build();
var configured = container.Resolve<RequiresDependency>();
The accepted answer is a terrible solution by the way, at least check the chaps who actually implemented the pattern.

public class Globals
{
private string setting1;
private string setting2;
#region Singleton Pattern Implementation
private class SingletonCreator
{
internal static readonly Globals uniqueInstance = new Globals();
static SingletonCreator()
{
}
}
/// <summary>Private Constructor for Singleton Pattern Implementaion</summary>
/// <remarks>can be used for initializing member variables</remarks>
private Globals()
{
}
/// <summary>Returns a reference to the unique instance of Globals class</summary>
/// <remarks>used for getting a reference of Globals class</remarks>
public static Globals GetInstance
{
get { return SingletonCreator.uniqueInstance; }
}
#endregion
public string Setting1
{
get { return this.setting1; }
set { this.setting1 = value; }
}
public string Setting2
{
get { return this.setting2; }
set { this.setting2 = value; }
}
public static int Constant1
{
get { reutrn 100; }
}
public static int Constat2
{
get { return 200; }
}
public static DateTime SqlMinDate
{
get { return new DateTime(1900, 1, 1, 0, 0, 0); }
}
}

I like this pattern, although it doesn't prevent someone from creating a non-singleton instance. It can sometimes can be better to educate the developers in your team on using the right methodology vs. going to heroic lengths to prevent some knucklehead from using your code the wrong way...
public class GenericSingleton<T> where T : new()
{
private static T ms_StaticInstance = new T();
public T Build()
{
return ms_StaticInstance;
}
}
...
GenericSingleton<SimpleType> builder1 = new GenericSingleton<SimpleType>();
SimpleType simple = builder1.Build();
This will give you a single instance (instantiated the right way) and will effectively be lazy, because the static constructor doesn't get called until Build() is called.

What you are describing is merely static functions and constants, not a singleton. The singleton design pattern (which is very rarely needed) describes a class that is instantiated, but only once, automatically, when first used.
It combines lazy initialization with a check to prevent multiple instantiation. It's only really useful for classes that wrap some concept that is physically singular, such as a wrapper around a hardware device.
Static constants and functions are just that: code that doesn't need an instance at all.
Ask yourself this: "Will this class break if there is more than one instance of it?" If the answer is no, you don't need a singleton.

hmmm... Few constants with related functions... would that not better be achieved through enums ? I know you can create a custom enum in Java with methods and all, the same should be attainable in C#, if not directly supported then can be done with simple class singleton with private constructor.
If your constants are semantically related you should considered enums (or equivalent concept) you will gain all advantages of the const static variables + you will be able to use to your advantage the type checking of the compiler.
My 2 cent

Personally I would go for a dependency injection framework, like Unity, all of them are able to configure singleton items in the container and would improve coupling by moving from a class dependency to interface dependency.

You can make a simple manual static singleton implementation for your common (non-static) class by adding a static property Instance (name can vary) into it with initialization like this:
public class MyClass
{
private static MyClass _instance;
public static MyClass Instance => _instance ?? (_instance = new MyClass());
// add here whatever constructor and other logic you like or need.
}
Then it can be resolved anywhere from this namespace like this:
var myClass = MyClass.Instance; // without any new keyword
myClass.SomeNonStaticMethod();
// or:
MyClass.Instance.SomeNonStaticMethod();
// or:
MyClass.Instance.SomeNonStaticProperty = "new value";

By hiding public constructor, adding a private static field to hold this only instance, and adding a static factory method (with lazy initializer) to return that single instance
public class MySingleton
{
private static MySingleton sngltn;
private static object locker;
private MySingleton() {} // Hides parameterless ctor, inhibits use of new()
public static MySingleton GetMySingleton()
{
lock(locker)
return sngltn?? new MySingleton();
}
}

I have written a class for my project using Singleton pattern. It is very easy to use. Hope it will work for you. Please find the code following.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
namespace TEClaim.Models
{
public class LogedinUserDetails
{
public string UserID { get; set; }
public string UserRole { get; set; }
public string UserSupervisor { get; set; }
public LogedinUserDetails()
{
}
public static LogedinUserDetails Singleton()
{
LogedinUserDetails oSingleton;
if (null == System.Web.HttpContext.Current.Session["LogedinUserDetails"])
{
oSingleton = new LogedinUserDetails();
System.Web.HttpContext.Current.Session["LogedinUserDetails"] = oSingleton;
}
else
{
oSingleton = (LogedinUserDetails)System.Web.HttpContext.Current.Session["LogedinUserDetails"];
}
//Return the single instance of this class that was stored in the session
return oSingleton;
}
}
}
Now you can set variable value for the above code in your application like this..
[HttpPost]
public ActionResult Login(FormCollection collection)
{
LogedinUserDetails User_Details = LogedinUserDetails.Singleton();
User_Details.UserID = "12";
User_Details.UserRole = "SuperAdmin";
User_Details.UserSupervisor = "815978";
return RedirectToAction("Dashboard", "Home");
}
And you can retrieve those value like this..
public ActionResult Dashboard()
{
LogedinUserDetails User_Details = LogedinUserDetails.Singleton();
ViewData["UserID"] = User_Details.UserID;
ViewData["UserRole"] = User_Details.UserRole;
ViewData["UserSupervisor"] = User_Details.UserSupervisor;
return View();
}

In c# it could be (Thread safe as well as lazy initialization):
public sealed class MySingleton
{
static volatile Lazy<MySingleton> _instance = new Lazy<MySingleton>(() => new MySingleton(), true);
public static MySingleton Instance => _instance.Value;
private MySingleton() { }
}

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