For my recent project which my colleague was in charge, I faced a question on How to restrict a class from creating multiple instances.
After five minutes of R&D I came across a word "Singleton". I referred a bulky book from shelf and rolled down until this word signalled my brain "Singleton Pattern".
1) What is a singleton pattern?
2) Why is it used for?
3) How to implement?
1) What is singleton pattern?
A: In software engineering this is a design pattern used to restrict instantiation of a class to one object. In another point of view, we make one and only one instance of a class during runtime. The same instance will be resused throughout the life cycle untile exclusively destroyed.
2) Why is it used for?
A: Depends on the requirement.
After five minutes of R&D I came across a word "Singleton". I referred a bulky book from shelf and rolled down until this word signalled my brain "Singleton Pattern".
1) What is a singleton pattern?
2) Why is it used for?
3) How to implement?
1) What is singleton pattern?
A: In software engineering this is a design pattern used to restrict instantiation of a class to one object. In another point of view, we make one and only one instance of a class during runtime. The same instance will be resused throughout the life cycle untile exclusively destroyed.
2) Why is it used for?
A: Depends on the requirement.
In my application the singleton was used to keep only one instance until EOD(End Of Day). For which I used a static (C#.Net)/shared(VB.Net) method instead of giving access to constructor.
eg: If you want all the users to use the only instance of a class for all purposes(transactions in my case), this is the right choice.
3) How to Implement?
A: Remember the following points
- Declare the default and all overridden constructors with private access specifier.
- Create a static(C#.Net)/shared(VB.Net) function which will take care of instantiation of a new object.
- You use only the static method to create a new instance, use of keyword 'new' will be restricted in this case.
In the windows form there are two buttons
a) Get : Calls the unique instance method.
b) Destroy : Destroy the unique instance created for a new creation.
My Singleton class
public class SingletonClass
{
private static SingletonClass MySingletonClass;
string _szValue = "";
//use this if you have any custom constructor
private SingletonClass(string szValue)
{
_szValue = szValue;
}
//This is how you restrict an instance is created outside this class using keyword NEW
private SingletonClass()
{
}
public string szDate
{
get;
set;
}
///The function handles creation of a new instance of this class.
///A new instance is created only if there is no previous instances created.
public static SingletonClass UniqueInstance(string szValue)
{
if (MySingletonClass == null)
{
MySingletonClass = new SingletonClass(szValue);
MySingletonClass.szDate = DateTime.Now.ToString();
}
return MySingletonClass;
}
public static void DestroyInstance()
{
MySingletonClass = null;
}
}
Form1 code behind
///Button click event of Get.
///Returns date at which instance was initially created.
private void btnGet_Click(object sender, EventArgs e)
{
///The most important point is this.
///Instead of declaring it using keyword new, you call the function.
///ie, we are avoiding SingletonClass mysingleton = new SingletonClass()
SingletonClass mysingleton = SingletonClass.UniqueInstance("hello");
MessageBox.Show(mysingleton.szDate);
}
private void btnDestroy_Click(object sender, EventArgs e)
{
SingletonClass.DestroyInstance();
}
