CODEWITHSUNDEEP
c#singleton
Devin Andres Salemi
Devin Andres Salemi

Reputation: 2206

Can a singleton be made without a private constructor?

    private static PostalCodeInfoTable postalCodeInfoTable = new PostalCodeInfoTable();

    public static PostalCodeInfoTable GetInstance() { return postalCodeInfoTable; }


    public PostalCodeInfoTable()
    {
        db = new GetConnectionString());
    }

I use the GetInstance() to create a PostalCodeInfoTable, then I use a member function to load some data that I haven't shown here, then I use GetInstance again and then data is already pre-loaded, so it seems we have a singleton, but I'm confused theoretically why we can do this with a public constructor

Upvotes: 0

Views: 3014

Answers (5)

Jeff W
Jeff W

Reputation: 59

You could use an enum like so:

public enum PostalCodeInfoTable
{
    Instance
}

You can then access this instance using:

var postalCodeInfoTable = PostalCodeInfoTable.Instance;

The Instance value within the enum implicitly represents the single instance of PostalCodeInfoTable. The C# runtime guarantees that enum values are initialized only once, making this a thread-safe way to implement a singleton.

EDIT: Can whoever has marked me down please tell me what is wrong with my solution? An enum is a class, and this is a valid way of creating a singleton without an explicit private constructor.

Upvotes: -1

Zzz
Zzz

Reputation: 3025

A singleton cannot be made without a private constructor!

If a class has a public constructor, then anybody can create an instance of it at any time, that would make it not a singleton.

In order for it to be a singleton there can only be one instance.

From wikipedia

An implementation of the singleton pattern must:

  1. ensure that only one instance of the singleton class ever exists;
  2. and provide global access to that instance.

Typically, this is done by:

  1. declaring all constructors of the class to be private;
  2. and providing a static method that returns a reference to the instance.

Upvotes: 7

Christopher
Christopher

Reputation: 9804

What you created here is a static variable (PostalCodeInfoTable.postalCodeInfoTable) that can only be accessed through a static method (PostalCodeInfoTable.GetInstance()).

The PostalCodeInfoTable instance is created by the TypeConstructor, wich is rarely used outside of runtime constant declaration.

The public constructor? That one is totally irrelevant. I am not sure what purpose it has. It can not affect what is int the static field. Indeed all it does is break the single pattern:

var temp1 = PostalCodeInfoTable.GetInstance();
var temp2 = new PostalCodeInfoTable();

//This will return false, as they are not the same instances
object.ReferenceEquals(temp1, temp2);

Upvotes: 1

Markiian Benovskyi
Markiian Benovskyi

Reputation: 2161

An implementation of the singleton pattern must:

  • ensure that only one instance of the singleton class ever exists;
  • provide global access to that instance.

Typically, this is done by:

  • declaring all constructors of the class to be private;
  • providing a static method that returns a reference to the instance.

The instance is usually stored as a private static variable; the instance is created when the variable is initialized, at some point before the static method is first called.

Correct example:

public class PostalCodeInfoTable{

    //.........

    private static PostalCodeInfoTable Instance = new PostalCodeInfoTable();

    private PostalCodeInfoTable() {
        db = new GetConnectionString();
    }

    public static PostalCodeInfoTable GetInstance() {
        return Instance;
    }
}

Upvotes: 0

RMH
RMH

Reputation: 837

A singleton is nothing more that an instance of a class that is expected to have only a single instance. Usually you would hide the constructor by making it private and using a static member to access it, making sure only a single instance can exist within your application.

As the constructor is public, you can still create a new instance. Making it no longer a 'Singleton' by definition, however this does not mean it cannot behave as one.

Upvotes: 2

Related Questions