Question on Private Constructor

Question

0.00/5 (No votes)

See more:

PRIVATE Constructor is use in .net or Not?
if YES then in Which Situation we use PRIVATE Constructor?
give me one example?

Posted 25-Jan-13 17:35pm

VISHWAS KAPTE

Add a Solution

3 solutions

Solution 3

look Private constructor[^]

Posted 25-Jan-13 21:51pm

Prosan

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Abhinav S · Accepted Answer · 2013-01-25T18:05:00

Solution 1

A private constructor can occasionally be used in .Net.
An example could be to generate a singleton class - a class which has only instance.

For e.g.

class A
{
  static A o;
  private A()
  {
  }

  public static A getinstance()
  {
    if (o == null) 
    { 
       return o = new A();
    }
    else
    {
       return o;
    }
  }
}

Posted 25-Jan-13 18:05pm

Abhinav S

Updated 25-Jan-13 21:24pm

v4

Comments

Sergey Alexandrovich Kryukov 26-Jan-13 0:23am

Aha, a 5, but will you format its indentation to make it readable. Just a note: this is not a perfect singleton, which should not return its instance. For better singleton, please see:

http://csharpindepth.com/Articles/General/Singleton.aspx

But it illustrates the idea well.
—SA

Abhinav S 26-Jan-13 3:27am

Thank you. Yes I have been through that article before. The Lazy<t> initialization is a new addition - though I'm not familiar with the Lazy keyword.

Menon Santosh 26-Jan-13 1:28am

my +5

Abhinav S 26-Jan-13 3:20am

Thank you.

Sergey Alexandrovich Kryukov · Accepted Answer · 2013-01-25T18:08:00

Private constructor is used to disable construction of a class using a constructor. It makes sense for a non-static class, of course. Let's set aside static constructors: they do not allow access modifiers at all, and are always treated as private.

Here is why: if no constructors defined, one default implicitly defined constructor still allows to instantiate this class. Naturally, this constructor is parameterless. So, defining a private parameterless constructor makes it impossible to construct an instance of a class. Other private constructors can be also added.

Now, why? Apparently, if an object cannot be constructed outside of the class, it can be constructed inside. So the purpose of all this activity is actually to create a factory method which returns the instantiated instance of the same class. Apparently, even though all constructors are private, inside class construction can be performed.

Why not a usual way, with non-private constructors? By different reasons an author of code designer may think of, but one of the is essential: because such factory method might return instances of some derived classes. So, compile-time return type is of the same class (which therefore can also be abstract), but run-time types returned are derived (and, of course, never abstract).

For a good example, look at this FCL type: System.Net.WebRequest. It has no constructors at all, but it has a factory method Create. How it works? It does not have public or internal constructor (sorry, not a perfect example: the constructors are not private, but it's still interesting to get an idea). Instead, you use factory methods. It's most interesting to consider the factory method Create(string). It is supposed to be a URI string, and the actual run-time type is determined by the URI scheme (http://en.wikipedia.org/wiki/URI_scheme[^]). Depending of the scheme detected, one of the derived types is constructed and returned: HttpWebRequest, HttpFtpRequest, etc.

Please see:
http://msdn.microsoft.com/en-us/library/system.net.webrequest.aspx[^].

—SA