How to create only one instance per class

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Even after googling I didn't find anything exactly fitting!

Please help me in this.

Modification:

Can somebody please explain this line in the Singleton pattern:

C#

public class OnlyOne
{
    // Please explain this line too me
    private static OnlyOne instance = null;

    private OnlyOne()
    {
    }

    public static OnlyOne getInstance()
    {
        if(instance==null)
        {
            instance = new OnlyOne();
        }
        return instance;
    }
}

Posted 14-Mar-11 23:08pm

JJamesCChowdare

Updated 15-Mar-11 0:03am

Manfred Rudolf Bihy

v6

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Manfred Rudolf Bihy 15-Mar-11 5:51am

Please see my adapted solution. I put a hint in there that will help you.

Tarun.K.S 15-Mar-11 5:53am

The constructor is private, so you cannot create an object of OnlyOne class. There is a getInstance method which you have to call to return the instance. Note that it will be created only once.

Manfred Rudolf Bihy 15-Mar-11 6:01am

Edit: Spelling and grammar. Tagged correctly. Added question extension OP had posted as a solution.

Albin Abel 15-Mar-11 14:50pm

I think OP can accept all of the answers below

4 solutions

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Tarun.K.S · Accepted Answer · 2011-03-14T23:14:00

Solution 1

That's possible using a Singleton Class.
Check this link :

http://csharpindepth.com/Articles/General/Singleton.aspx[^]

The author provided a nice and elegant explanation to implement it, its thread safe too.

Hope it helped! :)

Posted 14-Mar-11 23:14pm

Tarun.K.S

Updated 14-Mar-11 23:17pm

v2

Comments

Manfred Rudolf Bihy 15-Mar-11 5:40am

Singleton is the way to go! 5+

Tarun.K.S 15-Mar-11 5:49am

Thanks! :)

Dalek Dave 15-Mar-11 5:45am

Good Link

Tarun.K.S 15-Mar-11 5:50am

Thanks :)

Albin Abel 15-Mar-11 13:24pm

Thread safe is a good measure for web applications though it has little over head. My 5

Tarun.K.S 15-Mar-11 14:28pm

Thanks Albin! :)
Do visit GIT!

Sergey Alexandrovich Kryukov 15-Mar-11 15:39pm

Sorru, what about GIT?
--SA

Tarun.K.S 16-Mar-11 2:11am

Actually i was talking about the Indian forum here : http://www.codeproject.com/Forums/1580229/Hindi.aspx
Do visit it!

Sergey Alexandrovich Kryukov 16-Mar-11 3:24am

Ha! Separate Indian forum (I heard of it), all in English, I guess...
Thank you.

Tarun.K.S 16-Mar-11 4:23am

Hey you didn't notice it, I forgot to capitalize the "i" in the above comment! lol

Sergey Alexandrovich Kryukov 16-Mar-11 4:57am

I did. It looks like Indian "accent" to me, don't you know why? In Russian, this is only one letter, but lo-case. Russian mistakes are very different: not capitalization, but, say, swapping "he" and "she", very hard to say why? Extra modesty: lo-case I but upper-case "Sir", which we never use, even in US (Russian genders a very distinct by the word ending. My daughter has different form of family name because of the gender form, even her US name. I do not adjust to American custom, have them accept ours :-)
--SA

Tarun.K.S 16-Mar-11 7:20am

Ya I was checking in GIT and I found that they were using "I" instead of "i", my bad! I will be careful next time. Lol swapping "he" and "she"! But I didn't get that gender part.
SA wrote : "I do not adjust to American custom, have them accept ours :-) "
That's cool! :)

Sergey Alexandrovich Kryukov 16-Mar-11 12:54pm

Gender part is: there 3 genders, easily recognized by ending: male sound hard (ending with consonant), female sound soft, ending with 'a' + some other forms (soft sign at the end), "middle gender" adding with 'o' or 'ja', etc. Even flexible family names should be gender-flexed (most of the Slavic ones; native Russian names are mostly Slavic while given names are almost all Biblical) are English 'she' or 'he' sound neutral and therefore should not confuse Russians (they are not counter-suggestive to the Russian pattern I sketched above, and Russians perfectly know the difference), so it's hard to understand why do I and other mix them up in fast speech.
Don't get me wrong about American custom. I mean many Russians here give up Russian language (especially those who cannot learn good English, so this sort of people end up having no good cultured language at all), and simplify their official names (for example, give up gender flexes) into American English style. I think this is stupid. A different culture is different thing. In my observation, people understand foreign culture better if they know and respect their original.
--SA

Tarun.K.S 16-Mar-11 14:36pm

Now it makes sense.
Like you gave the example above of your daughter, Masha(ending with a).
You quoted this very well sir,
"people understand foreign culture better if they know and respect their original." Well said! people should stick to their culture.
Wow i learned many things here.:)
Thanks SA for sharing this!

Sergey Alexandrovich Kryukov 16-Mar-11 17:02pm

There are about 100 forms of here (and my) name, counting all flexes and grammar cases. Full name Maria :-) Hey, let's move to private...
--SA

Tarun.K.S 17-Mar-11 2:44am

Whoa that's huge!

Sergey Alexandrovich Kryukov 17-Mar-11 3:16am

:-) This is including pet forms, humorous, all cases (*6), etc.
--SA

Sergey Alexandrovich Kryukov 16-Mar-11 12:56pm

Look, can you imagine how we irritate OP. :-)
(Sorry, JJames!)
If you want to talk, use http://www.SAKryukov.org/contact/
--SA

Tarun.K.S 16-Mar-11 14:40pm

Hahaha Man i talk and question a lot! :P
I shall definitely contact you through your site, or even better i can catch you on facebook! :D
Thanks again SA for the wealth of info!

Tarun.K.S 16-Mar-11 14:42pm

Damn I still forgot to capitalize "i". When will "I" improve! :D

Sergey Alexandrovich Kryukov 16-Mar-11 15:23pm

Thank you, please not yet another social site! Please write via my site. As an option, we can share Skype accounts, how about that?
--SA

Tarun.K.S 17-Mar-11 2:41am

Well i don't have a Skype account, but once i make one i shall share it, till then i will write through your site!

Sergey Alexandrovich Kryukov 17-Mar-11 3:13am

My site, e-mail will be fine.
--SA

Sergey Alexandrovich Kryukov 15-Mar-11 15:38pm

Yes, the article looks good, 5 for the reference.
--SA

Tarun.K.S 16-Mar-11 2:13am

Thanks SA!

Espen Harlinn 17-Mar-11 10:06am

Nice and simple, my 5

Sandeep Mewara · Accepted Answer · 2011-03-14T23:26:00

Solution 2

Okay. Did you come across ~~class~~ the design pattern called Singleton class yet?

Here:
http://msdn.microsoft.com/en-us/library/ff650316.aspx[^]
http://msdn.microsoft.com/en-us/library/ee817670.aspx[^]

Posted 14-Mar-11 23:26pm

Sandeep Mewara

Updated 14-Mar-11 23:38pm

v2

Comments

Manfred Rudolf Bihy 15-Mar-11 5:39am

If you don't mind I changed the first "class" to design pattern.
Otherwise 5+

Sandeep Mewara 15-Mar-11 6:01am

Not at all. Infact, thanks. :)

Albin Abel 15-Mar-11 13:22pm

Good informative links. My 5

Sergey Alexandrovich Kryukov 15-Mar-11 15:39pm

Those references look useful, too, a 5.
--SA

Espen Harlinn 17-Mar-11 10:06am

Good links, my 5

Manfred Rudolf Bihy · Accepted Answer · 2011-03-14T23:37:00

Please see my solution to this question here: how to open only one instance of class not more than that ?[^]. Eventhough it's for a windows form class the same principles apply for other classes too.
As Sandeep already pointed out there is a design pattern called Singleton, which will solve your problem.

To give you a hint:
The object creation from outside of the class is prohibited by making the constructor private. Anything marked private can only be accessed from within the class, but not from the outside.

Best Regards,

Espen Harlinn · Accepted Answer · 2011-03-15T00:00:00

Solution 6

Take a look at the Lazy<T> Class[^] - it provides an implementation of the singleton pattern.

Your implementation contains a small bug - you need to make getInstance() static too. The will enable you to get the single instance of OnlyOne using OnlyOne obj = OnlyOne::getInstance();

private static OnlyOne instance = null;

private OnlyOne()    
{
}    

public static OnlyOne getInstance()    
{        
 if(instance==null)
 {
  instance = new OnlyOne();
 }
 return instance;
}

From the documentation[^]:
Use the static modifier to declare a static member, which belongs to the type itself rather than to a specific object. The static modifier can be used with fields, methods, properties, operators, events and constructors, but cannot be used with indexers, destructors, or types.

Regards
Espen Harlinn

Posted 15-Mar-11 0:00am

Espen Harlinn

Comments

Manfred Rudolf Bihy 15-Mar-11 6:26am

You're correct! 5+
Fixed it.

Espen Harlinn 15-Mar-11 7:23am

Thanks for the vote Manfred!

Manfred Rudolf Bihy 15-Mar-11 8:27am

You're welcome!

Albin Abel 15-Mar-11 13:19pm

Yes Lazy instantiation is a very good method with singleton and it has its thread safe. My 5+

Espen Harlinn 15-Mar-11 14:17pm

Thanks for the vote AlbinAbel!

Sergey Alexandrovich Kryukov 15-Mar-11 15:42pm

Lazy add some laziness... I mean elegance, thanks the introduction of this genetic.
You answer is good: short and simple. 5++++
--SA

Espen Harlinn 15-Mar-11 15:44pm

Thanks SAKryukov!

JF2015 16-Mar-11 4:23am

Very good and easy to understand answer.

Espen Harlinn 16-Mar-11 13:25pm

Thank you, JF2015!

JJamesCChowdare 14-Mar-16 12:41pm

Thank you, frends,

How to create only one instance per class

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