Implementing Design Patterns in C# - Singleton Pattern

The constructor is made private and used to initialize the numOfReference and default value of code. GetInstance() method checks the instance, if it is null then it assign it an instance of Singleton otherwise return the old reference. GetInstance() also increments the numOfReference each time it is called to keep track of current number of reference to our shared instance. Property Reference returns the number of reference referencing our shared object while Code property can be used to set/get the code string of shared object.

The second case Singleton class (Singleton2) looks like:

class Singleton2
{
private static int numOfInstance = 0;
public Singleton2()
{
if(numOfInstance == 0)
{
Console.WriteLine("\r\nCreating First Object of Singleton2 class...");
numOfInstance++;
}
else
{
throw new Exception("This class is Singleton,
+ so only one object of it can be instantiated.");
}
}
}

Here we make the constructor public and use a private field numOfInstance which is incremented for each constructor call. If numOfInstance is zero (no object is yet instantiated), a new object is allowed to made. But, if this value is not zero (there is already an object of Singleton2 class, an exception is thrown.

Now let us try these with Main() method, the code in main is like:

static void Main(string[] args)
{
Singleton obj1 = Singleton.GetInstance();
obj1.Code = "Faraz Rasheed";
Console.WriteLine("No. of references : " + Singleton.Reference);
Console.WriteLine("First Objects code: " + obj1.Code);
Singleton obj2 = Singleton.GetInstance();
Console.WriteLine("No. of references : " + Singleton.Reference);
Console.WriteLine("Second Objects code: " + obj2.Code);
Singleton2 obj3 = new Singleton2();
Singleton2 obj4 = new Singleton2();
}

First we called GetInstance() and took the reference to the new object in obj1. The code for this object is changed to Faraz Rasheed which was Maasoom Faraz by default. Then, we check for number of references to this object and code set for this. Next, we get another reference to this object through GetInstance() in obj2. We again check the number of references and code using this reference. If both the references are pointing to the same shared object, the number of references now should be 2 while the code should also be Faraz Rasheed (remember if obj2 points to a new object then code would have been Maasoom Faraz, the default one) which is shown in the output.

We also made an object of Singleton2. When constructor is called for the first reference (obj3), the new object is instantiated printing the message on Console. But when another instance of Singleton2 is attempted to be made through obj4, an exception is thrown saying Singleton class can only be instantiated once.

Sample output from the program is:

No. of references : 1
First Objects code: Faraz Rasheed
No. of references : 2
Second Objects code: Faraz Rasheed

Creating First Object of Singleton2 class...

Unhandled Exception: System.Exception: This class is Singleton,so only one object of it can be instantiated.
at Singleton.Singleton2..ctor() in c:\documents and settings\msff\my documents\visual studio projects\singleton\class1.cs:line 75
at Singleton.Client.Main(String[] args) in c:\documents and settings\msff\my
documents\visual studio projects\singleton\class1.cs:line 20

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The complete source code is also attached with the article. Take a look at it to understand it to full.

Benefits Achieved Through This Pattern:

By using Singleton Design Pattern, it is possible for a programmer to control the instantiation of class by allowing only single object to be instantiated when desirable.