Unit Test for Singleton Pattern?

Question

Suppose, I am using a package p1 with classes A and B. I do not have access to the implementation of package p1.

Now, in my application I want only one instance of A at any time.

I have created a singleton class C in my application for class A.

Sample:

public Class ClassC
{
    private static readonly ClassA singletonObj = new ClassA();

    private ClassC();

    public static ClassA ClassC
    {
       get
       {
          return singletonObj;
       }
    }
}

How to unit test on class C to ensure a single instance was created for my application?

Answer 1

If you want class A to be singleton, then your above code is irrelevent.
You should write class A like that:

 public Class ClassA { private static Singleton instance; private Singleton() {} public static Singleton Instance { get { if (instance == null) { instance = new Singleton(); } return instance; } } //Any other function & members related to ClassA } 

In Order to test the singletone, try to create few ClassA instances in your code. Good result is when on every creation of "new instance", you get the same instance, meaning - you can have 2 instances having duffrent data - they will all be kinda pointer to same instance.