Singleton implemented with C# could be like:
public class Singleton
{
private static Singleton instance;
private Singleton() {}
public static Singleton Instance
{
get
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
If I use static to implement it like:
public static class Globals{
public static Singleton Instance = new Singleton();
}
in this way, app should also only get the one instance for the entire app.
So what's the difference between these 2 approaches? Why not use static member directly(more simple and straight forward)?
If you use the second approach:
public static class Globals{
public static Singleton Instance = new Singleton();
}
There's nothing preventing somebody from doing:
Singleton anotherInstance = new Singleton(); // Violates singleton rules
You also don't get the same lazy initialization your first version (attempts to) achieve, plus you're using a public field, which doesn't allow you the same flexibility in the future if you need to change what happens when a value is fetched.
Note that .NET 4 provides a potentially better approach to making a singleton:
public class Singleton
{
private static readonly Lazy<Singleton> instance = new Lazy<Singleton>( ()=> new Singleton());
private Singleton() {}
public static Singleton Instance
{
get
{
return instance.Value;
}
}
}
This is nice because it's fully lazy and fully thread safe, but also simple.
Below are some differences between static and Singleton :
Singleton is a pattern while static is a keyword.
Singleton class can have static and non static method, but static class can have only static members and methods.
We can implement interface in a Singleton class but in static class we cannot implement interface.
We can extend the Singleton class while we static class we can’t, i.e. Singleton class can be derived from any type of class.
for more static vs Singleton
Related
I was implementing a singleton with static initialization like this:
internal sealed class MySingleton
{
private static readonly MySingleton instance = new MySingleton();
public static MySingleton Instance => instance;
private MySingleton()
{
// elided
}
}
ReSharper squiggled the public property Instance and offered to convert it to an auto-property. The result, if accepting the fix, looks like this:
internal sealed class MySingleton
{
public static MySingleton Instance { get; } = new MySingleton();
private MySingleton()
{
// elided
}
}
Doesn't this "fix" make it so that every call to the Instance property will generate a brand new MySingleton, thus defeating the whole purpose, or is there something about static properties that ReSharper knows and I don't?
It's a read-only auto property, using a syntax that was introduced in C# 6. The code on the right-hand side is executed only once, just to initially assign the value of the property.
As I go through the differences between Singleton Vs Static class, I came across one point that we can inherit an interface in singleton class and can call singleton through interface for multiple implementation.
I would like some code demonstration with good example, how object orientation can achieve through singleton and not through static.
Thanks,
Although it's hard to tell what exactly you are referring to, one pattern you might be referring to is the Multiton pattern, where you manage a map of named instances as key-value pairs.
That's basically a factory, but each instance is only created once:
I've modified the Wikipedia example a bit to show that you can even derive from a singleton class, as long as your concrete implementations are private and within the original class:
class FooMultiton
{
private static readonly Dictionary<object, FooMultiton> _instances =
new Dictionary<object, FooMultiton>();
// this is the classic good old singleton trick (prevent direct instantiation)
private FooMultiton()
{ }
// you can also have private concrete implementations,
// invisible to the outside world
private class ConcreteFooMultitonOne : FooMultiton
{ }
public static FooMultiton GetInstance(object key)
{
lock (_instances)
{
FooMultiton instance;
// if it doesn't exist, create it and store it
if (!_instances.TryGetValue(key, out instance))
{
// at this point, you can create a derived class instance
instance = new ConcreteFooMultitonOne();
_instances.Add(key, instance);
}
// always return the same ("singleton") instance for this key
return instance;
}
}
}
Also, generally, if a singleton is not a static class, it can implement any interface you want. The only thing that a singleton pattern prevents is instantiation of multiple instances of a singleton class, but that doesn't mean you cannot completely replace the implementation with something else.
For example, if you have a singleton which is not a static class:
interface ICanTalk
{
string Talk();
}
class Singleton : ICanTalk
{
private Singleton() { }
private static readonly Singleton _instance = new Singleton();
public static Singleton Instance
{ get { return _instance; } }
public string Talk()
{ return "this is a singleton"; }
}
You can also have a number of different implementations:
class OtherInstance : ICanTalk
{
public string Talk()
{ return "this is something else"; }
}
Then you are free to choose any implementation you want, but get only a single instance of the Singleton class:
ICanTalk item;
item = Singleton.Instance;
item = new OtherInstance();
item = new YetAnotherInstance();
According to nkr1pr
Every class can implement an interface, and a Singleton is just a "normal" class that makes sure that only one instance of it exists at any point in time apart from the other business logic it may implement. This also means that a Singleton has at least 2 responsibities and this is not good OO design as classes should only have 1 responsibility and make sure they are good at that responsibility, but that is another discussion.
Something like:
public interface MyInterface
{
}
And
public class MySingleton:MyInterface
{
private static MyInterface instance = new MySingleton();
private MySingleton()
{
}
public static MyInterface getInstance()
{
return instance;
}
}
I'm not sure what you are asking, but singleton classes can implement interfaces.
singleton class does not mean static class, one of the method to create a singleton instance is
to make use of static members.
public class MyInterfaceImplementation : IMyInterface
{
private static MyInterfaceImplementation instance;
private static readonly object lockObj = new object();
private MyInterfaceImplementation() { } //private .ctor
public static MyInterfaceImplementation Instance
{
get
{
if (instance == null)
{
lock (lockObj)
{
instance = new MyInterfaceImplementation();
}
}
return instance;
}
}
public void MyInterfaceMethod()
{
//Implement here
}
}
I have a class which is lazy instantiated by another library. I don't have control over that library code but still need to be sure it cannot create more than one instance of my class.
Is it possible ? how ?
Some simple solutions to this question hides some problems, for a full understanding of this matter I recommend the reading of the following article
http://www.yoda.arachsys.com/csharp/singleton.html
Which concludes with an optimal solution
public sealed class Singleton
{
Singleton()
{
}
public static Singleton Instance
{
get
{
return Nested.instance;
}
}
class Nested
{
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Nested()
{
}
internal static readonly Singleton instance = new Singleton();
}
}
If it's calling your class's constructor, the only thing you can do is throw an exception in the constructor if you determine that an instance was previously created. Since you don't have control over the other library, you wouldn't be able to use a factory method or static property which would normally be the way you control access to a singleton.
Depending on your situation, you may also look into using a lightweight proxy that wraps a singleton instance. In this example, any number of MyObjectProxy can be created but they all defer their implementation to a single instance of MyObject.
public class MyObject {
internal MyObject() {
}
private int _counter;
public int Increment() {
return Interlocked.Increment(ref _counter);
}
}
public class MyObjectProxy {
private static readonly MyObject _singleton = new MyObject();
public MyObjectProxy() {
}
public int Increment() {
return _singleton.Increment();
}
}
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I need to store a bunch of variables that need to be accessed globally and I'm wondering if a singleton pattern would be applicable. From the examples I've seen, a singleton pattern is just a static class that can't be inherited. But the examples I've seen are overly complex for my needs. What would be the very simplest singleton class? Couldn't I just make a static, sealed class with some variables inside?
Typically a singleton isn't a static class - a singleton will give you a single instance of a class.
I don't know what examples you've seen, but usually the singleton pattern can be really simple in C#:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
static Singleton() {} // Make sure it's truly lazy
private Singleton() {} // Prevent instantiation outside
public static Singleton Instance { get { return instance; } }
}
That's not difficult.
The advantage of a singleton over static members is that the class can implement interfaces etc. Sometimes that's useful - but other times, static members would indeed do just as well. Additionally, it's usually easier to move from a singleton to a non-singleton later, e.g. passing in the singleton as a "configuration" object to dependency classes, rather than those dependency classes making direct static calls.
Personally I'd try to avoid using singletons where possible - they make testing harder, apart from anything else. They can occasionally be useful though.
There are several Patterns which might be appropriate for you, a singleton is one of the worse.
Registry
struct Data {
public String ProgramName;
public String Parameters;
}
class FooRegistry {
private static Dictionary<String, Data> registry = new Dictionary<String, Data>();
public static void Register(String key, Data data) {
FooRegistry.registry[key] = data;
}
public static void Get(String key) {
// Omitted: Check if key exists
return FooRegistry.registry[key];
}
}
Advantages
Easy to switch to a Mock Object for automated testing
You can still store multiple instances but if necessary you have only one instance.
Disadvantages
Slightly slower than a Singleton or a global Variable
Static Class
class GlobalStuff {
public static String ProgramName {get;set;}
public static String Parameters {get;set;}
private GlobalStuff() {}
}
Advantages
Simple
Fast
Disadvantages
Hard to switch dynamically to i.e. a Mock Object
Hard to switch to another object type if requirements change
Simple Singleton
class DataSingleton {
private static DataSingleton instance = null;
private DataSingleton() {}
public static DataSingleton Instance {
get {
if (DataSingleton.instance == null) DataSingleton.instance = new DataSingleton();
return DataSingleton;
}
}
}
Advantages
None really
Disadvantages
Hard to create a threadsafe singleton, the above Version will fail if multiple threads access the instance.
Hard to switch for a mock object
Personally I like the Registry Pattern but YMMV.
You should take a look at Dependency Injection as it's usually considered the best practice but it's too big a topic to explain here:
Dependency Injection
A Singleton isn't just a static class that can't be inherited. It's a regular class that can be instantiated only once, with everybody sharing that single instance (and making it thread safe is even more work).
The typical .NET code for a Singleton looks something like the following. This is a quick example, and not by any means the best implementation or thread-safe code:
public sealed class Singleton
{
Singleton _instance = null;
public Singleton Instance
{
get
{
if(_instance == null)
_instance = new Singleton();
return _instance;
}
}
// Default private constructor so only we can instanctiate
private Singleton() { }
// Default private static constructor
private static Singleton() { }
}
If you're going to go down the path you're thinking, a static sealed class will work just fine.
Using C# 6 Auto-Property Initializers.
public sealed class Singleton
{
private Singleton() { }
public static Singleton Instance { get; } = new Singleton();
}
Short and clean - I'll be happy to hear the downsides.
I know this Issue is old, but here is another solution using .Net 4.0 or later (including .Net Core and .Net Standard).
First, define your class that will be transformed into a Singleton:
public class ClassThatWillBeASingleton
{
private ClassThatWillBeASingleton()
{
Thread.Sleep(20);
guid = Guid.NewGuid();
Thread.Sleep(20);
}
public Guid guid { get; set; }
}
In this Example Class I've defined one constructor that Sleeps for a while, and then creates one new Guid and save to it's public property. (The Sleep is just for concurrency testing)
Notice that the constructor is private, so that no one can create a new instance of this class.
Now, We need to define the wrapper that will transform this class into a singleton:
public abstract class SingletonBase<T> where T : class
{
private static readonly Lazy<T> _Lazy = new Lazy<T>(() =>
{
// Get non-public constructors for T.
var ctors = typeof(T).GetConstructors(System.Reflection.BindingFlags.Instance |
System.Reflection.BindingFlags.NonPublic);
if (!Array.Exists(ctors, (ci) => ci.GetParameters().Length == 0))
throw new InvalidOperationException("Non-public ctor() was not found.");
var ctor = Array.Find(ctors, (ci) => ci.GetParameters().Length == 0);
// Invoke constructor and return resulting object.
return ctor.Invoke(new object[] { }) as T;
}, System.Threading.LazyThreadSafetyMode.ExecutionAndPublication);
public static T Instance
{
get { return _Lazy.Value; }
}
}
Notice that it uses Lazy to create a field _Lazy that knows how to instantiate a class using it's private constructor.
And it defines one Property Instance to access the Value of the Lazy field.
Notice the LazyThreadSafetyMode enum that is passed to the Lazy constructor. It is using ExecutionAndPublication. So only one thread will be allowed to initialize the Value of the Lazy field.
Now, all we have to do is define the wrapped class that will be a singleton:
public class ExampleSingleton : SingletonBase<ClassThatWillBeASingleton>
{
private ExampleSingleton () { }
}
Here is one example of the usage:
ExampleSingleton.Instance.guid;
And one test to assert that two threads will get the same instance of the Singleton:
[Fact()]
public void Instance_ParallelGuid_ExpectedReturnSameGuid()
{
Guid firstGuid = Guid.Empty;
Guid secondGuid = Guid.NewGuid();
Parallel.Invoke(() =>
{
firstGuid = Singleton4Tests.Instance.guid;
}, () =>
{
secondGuid = Singleton4Tests.Instance.guid;
});
Assert.Equal(firstGuid, secondGuid);
}
This test is calling the Value of the Lazy field concurrently, and we want to assert that both instances that will be returned from this property (Value of Lazy) are the same.
More details about this subject can be found at: C# in Depth
So, as far as I am concerned, this is the most concise and simple implementation of the Singleton pattern in C#.
http://blueonionsoftware.com/blog.aspx?p=c6e72c38-2839-4696-990a-3fbf9b2b0ba4
I would, however, suggest that singletons are really ugly patterns... I consider them to be an anti-pattern.
http://blogs.msdn.com/scottdensmore/archive/2004/05/25/140827.aspx
For me, I prefer to have something like a Repository, implementing IRepository. Your class can declare the dependency to IRepository in the constructor and it can be passed in using Dependency Injection or one of these methods:
http://houseofbilz.com/archive/2009/05/02.aspx
Use your language features.
Mostly simple thread-safe implementation is:
public sealed class Singleton
{
private static readonly Singleton _instance;
private Singleton() { }
static Singleton()
{
_instance = new Singleton();
}
public static Singleton Instance
{
get { return _instance; }
}
}
...what would be the very simplest singleton class?
Just to add one more possible solution. The simplest, most straight forward and easy to use approach I can think of would be something like this:
//The abstract singleton
public abstract class Singleton<T> where T : class
{
private static readonly Lazy<T> instance = new Lazy<T>( CreateInstance, true );
public static T Instance => instance.Value;
private static T CreateInstance()
{
return (T)Activator.CreateInstance( typeof(T), true);
}
}
//This is the usage for any class, that should be a singleton
public class MyClass : Singleton<MyClass>
{
private MyClass()
{
//Code...
}
//Code...
}
//Example usage of the Singleton
class Program
{
static void Main(string[] args)
{
MyClass clazz = MyClass.Instance;
}
}
What do you guys think about this for a generic singleton?
using System;
using System.Reflection;
// Use like this
/*
public class Highlander : Singleton<Highlander>
{
private Highlander()
{
Console.WriteLine("There can be only one...");
}
}
*/
public class Singleton<T> where T : class
{
private static T instance;
private static object initLock = new object();
public static T GetInstance()
{
if (instance == null)
{
CreateInstance();
}
return instance;
}
private static void CreateInstance()
{
lock (initLock)
{
if (instance == null)
{
Type t = typeof(T);
// Ensure there are no public constructors...
ConstructorInfo[] ctors = t.GetConstructors();
if (ctors.Length > 0)
{
throw new InvalidOperationException(String.Format("{0} has at least one accesible ctor making it impossible to enforce singleton behaviour", t.Name));
}
// Create an instance via the private constructor
instance = (T)Activator.CreateInstance(t, true);
}
}
}
}
Creating a singleton class is just a few lines of code, and with the difficulty of making a generic singleton i always write those lines of code.
public class Singleton
{
private Singleton() {}
static Singleton() {}
private static Singleton _instance = new Singleton();
public static Singleton Instance { get { return _instance; }}
}
The
private static Singleton _instance = new Singleton();
line removes the need for locking, as a static constructor is thread safe.
Well, it isn't really singleton - since you can't control T, there can be as many T instances as you like.
(removed thread-race; noted the double-checked usage)
I've deleted my previous answer as I hadn't noticed the code which checks for non-public constructors. However, this is a check which is only performed at execution time - there's no compile-time check, which is a strike against it. It also relies on having enough access to call the non-public constructor, which adds some limitations.
In addition, it doesn't prohibit internal constructors - so you can end up with non-singletons.
I'd personally create the instance in a static constructor for simple thread safety, too.
Basically I'm not much of a fan - it's pretty easy to create singleton classes, and you shouldn't be doing it that often anyway. Singletons are a pain for testing, decoupling etc.
This is my point using .NET 4
public class Singleton<T> where T : class, new()
{
Singleton (){}
private static readonly Lazy<T> instance = new Lazy<T>(()=> new T());
public static T Instance { get { return instance.Value; } }
}
and it's using is following:
public class Adaptor
{
public static Adaptor Instance { get { return Singleton<Adaptor>.Instance;}}
}
Merging AndreasN answer and Jon Skeet's "Fourth version - not quite as lazy, but thread-safe without using locks" of a Singleton c# implementation, why don't use a code snippet to do all the hard work:
<CodeSnippets xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
<CodeSnippet Format="1.0.0">
<Header>
<Title>Singleton Class</Title>
<Author>TWSoft</Author>
<Description>Generates a singleton class</Description>
<SnippetTypes>
<SnippetType>Expansion</SnippetType>
</SnippetTypes>
<Keywords>
<Keyword>Singleton</Keyword>
</Keywords>
<Shortcut>singleton</Shortcut>
</Header>
<Snippet>
<Declarations>
<Literal>
<ID>ClassName</ID>
<ToolTip>Replace with class name</ToolTip>
<Default>MySingletonClass</Default>
</Literal>
</Declarations>
<Code Language="CSharp">
<![CDATA[
public class $ClassName$
{
#region Singleton
static readonly $ClassName$ mInstance = new $ClassName$();
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static $ClassName$()
{
}
private $ClassName$()
{
}
public static $ClassName$ Instance
{
get { return mInstance; }
}
#endregion
}
]]>
</Code>
</Snippet>
</CodeSnippet>
</CodeSnippets>
Then you can save this into a .snippt file, and add it to VS IDE (Tools->Code Snippets Manager)
Using generics is not useful for singletons. Because you can always create multiple instances of the type parameter T and then it is not by definition a singleton.
Look at this:
public sealed class Singleton<T> where T : class, new()
{
private static readonly Lazy<T> instance = new Lazy<T>(() => new T());
public static T Instance => instance.Value;
private Singleton() { }
}
And when you use it like this
public class Adapter
{
public static Adapter Instance => Singleton<Adapter>.Instance;
// private Adapter(){ } // doesn't compile.
}
you can still create Adapters yourself just call
new Adapter();
Adding the private constructor to Adapter breaks the code.
Note that this Adapter uses composition not inheritance. I.e. it is not a Singleton it has a Singleton. If something is a Singleton it should derive from or implement an interface.
public abstract class Singleton<T>
{
protected static Lazy<T> instance;
public static T Instance => instance.Value;
}
public sealed class Adapter : Singleton<Adapter>
{
static Adapter()
{
instance = new Lazy<Adapter>(() => new Adapter());
}
private Adapter() { }
}
Basically this only moves a static field into a generic base class while it is no longer readonly and therefore can be changed after initialization. Also it requires you to remember to add the private constructor, mark it as sealed and perform some initialization so it still isn't well encapsulated and prone to mistakes.
We can improve this by adding a check in the base constructor.
public abstract class Singleton<T> where T : Singleton<T>, new()
{
private static bool instantiated;
private static readonly Lazy<T> instance = new Lazy<T>(() => new T());
public static T Instance => instance.Value;
protected Singleton()
{
if (instantiated)
throw new Exception();
instantiated = true;
}
}
public /* sealed */ class Adapter : Singleton<Adapter>
{
}
Due to the fact that the Singleton is lazy the pattern is still broken.
new Adapter(); // this works
Adapter.Instance; // this throws an error.
also
Adapter.Instance; // this works
// just running in production someone decided to call:
new Adapter(); // this throws an error
These kind of error's can only be detected during testing.
I would still prefer to just use the pattern directly it's basically just 3 lines of code and it is compile time safe. Also when you need a different base classes but still need a lazy singleton and require it to be a true singleton a simple solution exists: (see: http://csharpindepth.com/Articles/General/Singleton.aspx)
public sealed class Adapter
{
private static readonly Lazy<Adapter> instance = new Lazy<Adapter>(() => new Adapter());
public static Adapter Instance { get { return instance.Value; } }
private Adapter() { }
}
You cannot refactor this properly into a generic singleton that cannot be misused like the above investigated approaches.