08 November 2012

Constructors and Destructors invocation sequence in inheritance

Sometime back I got a interview question, in inheritance class relationship, when you are trying to create a derived class instance which constructor will be invoked first?

Is it the base class constructor invoked first?

OR

Is it the derived class constructor invoked first?

Let's create a simple console application in C# language to demonstrate constructor invocation sequence/order:

namespace ConstructorInvocationSequenceExample
{
    class Program
    {
        static void Main(string[] args)
        {
            DerivedClass derivedClassObject = new DerivedClass();
            Console.ReadKey();
        }
    }
    public class BaseClass
    {
        public BaseClass()
        {
            Console.WriteLine("Base Class Constructor invoked.");
        }
    }
    public class DerivedClass : BaseClass
    {
        public DerivedClass()
        {
            Console.WriteLine("Derived Class Constructor invoked.");
        }
    }
}
Let's have a look at the C# sample code output below:

Base Class Constructor invoked.
Derived Class Constructor invoked.

After analyzing the example's output we can say:

Base class constructor is invoked first and then the derived class constructor.

It make sense because during a building in under a construction, first basement will be built then the the floors.

Let's create a plain console application example to demonstrate destructor invocation order:

namespace DestructorInvocationOrderExample
{
    class Program
    {
        static void Main(string[] args)
        {
            DerivedClass derivedClassObject = new DerivedClass();
            // Set the object to null and call 
            // the GC.Collect(), so that
            // destructors are invoked.
            derivedClassObject = null;
            GC.Collect();
            Console.ReadKey();
        }
    }
    public class BaseClass
    {
        ~BaseClass()
        {
            Console.WriteLine("Base Class Destructor invoked.");
        }
    }
    public class DerivedClass : BaseClass
    {
        ~DerivedClass()
        {
            Console.WriteLine("Derived Class Destructor invoked.");
        }
    }
}
Let's have look at the example output:

Derived Class Destructor invoked.
Base Class Destructor invoked.

After analyzing the example's output it is clear that:

Derived class destructor is invoked first and then the base class destructor.

It make sense because when building is being demolished, similar to Garbage collection - the floors will demolished first and then the basement.