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.
Related
Today, I have been reading about static class and private constructor.
Static Class - We cannot create an instance on the static class. we cannot inherit the static class. Only single instance is generated.
Private Constructor - We cannot create an instance. We cannot inherit. (I Don't know about how many instance is generated.)
I created two console application i.e. One for static Class, One for Private constructor.
Static Class Code
I understood single object in generated as constructor is called once.
Private Constructor Code
Now, I didn't understand that whether any object is generated or not.
I have two question.
Question 1. I didn't find any particular difference between Private constructor and Static class. Can you please suggest me that in which scenario where I should use Private Constructor and where should I use Static class as I can use both of them.
Question 2. If I use private constructor, how many objects is generated?
Thanks.
EDIT :
I think that people didn't understand my question.
I know that static constructor always call once on the first reference. Static constructor is used to initialize static members of the class.
Question 1. I have a situation : I need to create a class which cannot be instantiated.I can do this by either static class or private constructor. So my question is that "Is there any difference between both of them? which one I should use??"
Question 2. If I use private constructor, how many object is created? If answer is 0 then how private constructor's memory allocation works in the CLR. There is no memory allocation if I use private constructor.
Both of you examples you are calling static methods the difference between the two is that the first method is being called within a static class which cannot be instantiated. The second class could be instantiated however you did not choose to.
The static constructor in the first example is run automatically at runtime when it is needed, it is generally only run once.
The private constructor is never run as you never instantiated a testPrivateConstructor object. not because it is private.
The Edit:
Q1: If you need a class that cannot be instantiated use a static class. Only use a private constructor instead of a static class if you need to initialise static members (or a singleton pattern).
Q2: If you use a private constructor you can instantiate an instance of your class within the class itself so the number of objects that are created depend on how many times you instantiate new objects.
your mixing up a few different things here
a static class public static class MyClass can only contain static elements and never be initialised
a constructor (whether public or private) always creates in instance of the class the public or private only says the visibility of the constructor.
this is commonly used when implementing a singleton design
private MyClass()
{
}
private static MyClass _Singleton;
public static MyClass Singleton
{
get
{
if(_Singleton==null) _Singleton = new MyClass();
return _Singleton
}
}
}
the other is a Class Initialiser, this is a little confusing because its syntax is very similar to a constructor baring the adding of a static keyword and lack of parameters
static MyClass()
{
//configure static variables on first us only
b = //read value from file or other resource not avalable at compile time
a = b.Lenth; //can't be be done in class body as b would not have been initialised yet
}
private static int a;
private static string b;
ergo if your class can't be instantiated then you can only declare is as static nothing else will do that,
if you call a private constructor then every call creates an instance
a class initialiser can never be called its fired automatically on the first use of a class and does not create an instance
EDIT:
here is a revised version of your test program
public static class StaticClassExample
{
public static void ClassFunction()
{
Console.WriteList("This is a class function")
}
}
public static class InitialisedStaticClassExample
{
static InitialisedStaticClassExample()
{
Console.WriteList("This class has been initialised")
}
public static void ClassFunction()
{
Console.WriteList("This is a class function")
}
}
public class PrivateConstuctorClassExample
{
static PrivateConstuctorClassExample()
{
Console.WriteList("This class has been initialised")
}
private PrivateConstuctorClassExample()
{
Console.WriteList("This class has been Instantiated")
}
public static void ClassFunction()
{
Console.WriteList("This is a class function");
var instance = new PrivateConstuctorClassExample();
instance.InstanceFunction();
}
public void InstanceFunction()
{
Console.WriteList("This is a instance function")
}
}
Static constructor will be called first time when the class is
referenced. Static constructor is used to initialize static members
of the class.
In the non static class the private or public constructor will not be
called. Static members will not be initialized either by private or
public constructor.
see the below exmaple
class Program
{
static void Main(string[] args)
{
OnlyOne.SetMyName("I m the only one."); //static constructor will be called first time when the class will be referenced.
Console.WriteLine(OnlyOne.GetMyName());
NoInstance.SetMyName("I have private constructor"); //No constructor will be called when the class will be referenced.
Console.WriteLine(NoInstance.GetMyName());
Console.Read();
}
}
static class OnlyOne
{
static string name;
/// <summary>
/// This will be called first time when even the class will be referenced.
/// </summary>
static OnlyOne()
{
name = string.Empty;
Console.WriteLine("Static constructor is called");
}
public static string GetMyName()
{
return name;
}
public static void SetMyName(string newName)
{
name = newName;
}
}
public class NoInstance
{
static string name;
private NoInstance()
{
name = string.Empty;
Console.WriteLine("No instance private constructor is called");
}
public static string GetMyName()
{
return name;
}
public static void SetMyName(string newName)
{
name = newName;
}
}
}
Question 2: 0
Question 1: you created a static class with a static constructor, but you do not need the instance in your case, since the method is static too and the only thing you need to run a static method is the class definition - no instance.
your second class works also without construction of an instance - the static method does not need one. and since this is a private constructor only class methods could create the instance.
you would use the private constructor only in case of the singleton pattern i guess - without a local caller for private constructor it is rather useless.
Constructor of a class is called upon creation of instance of the class.
Static constructors are called on initialization of the class. Read this
In example 1, your static constructor is initialized the as you are accessing a static method of the class. This is relevant for a static or non-static class.
In example 2, your private constructor isn't static.
Your constructor wasn't called as there was no instance created.
It has to be called by creating an instance of the class.
Private constructors are used to control the construction and destruction of the instance of the class.
Since, only the class can call its constructor in this case. You need a static method to get the instance of the class
For example,
public class TestPrivateConstructor
{
private TestPrivateConstructor()
{
Console.WriteLine("Instance is created, Private Constructor called");
}
static TestPrivateConstructor _instance;
public static TestPrivateConstructor GetInstance()
{
if(_instance == null)
{
_instance = new TestPrivateConstructor();
}
return _instance;
}
public static void DisposeInstance()
{
if(_instance !=null)
{
_instance.Dispose();
_instance = null;
}
}
public void TestMethod()
{
Console.WriteLine("Test MEthod Called");
}
void Dispose()
{
//Do something
}
}
For the code above, try using this. Now your private constructor is called as you created an instance.
class Program
{
public static void Main(string[] args)
{
//Private constructor
TestPrivateConstructor.GetInstance()
}
}
Using the approach above, you can control the construction and destruction of the object.
If you still have any questions, please feel free to ask.
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
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.