C# Language Inheritance. Constructors' calls sequence


Example

Consider we have a class Animal which has a child class Dog

class Animal
{
    public Animal()
    {
        Console.WriteLine("In Animal's constructor");
    }
}

class Dog : Animal
{
    public Dog()
    {
        Console.WriteLine("In Dog's constructor");
    }
}

By default every class implicitly inherits the Object class.

This is same as the above code.

class Animal : Object
{
    public Animal()
    {
        Console.WriteLine("In Animal's constructor");
    }
}

When creating an instance of Dog class, the base classes's default constructor (without parameters) will be called if there is no explicit call to another constructor in the parent class. In our case, first will be called Object's constructor, then Animal's and at the end Dog's constructor.

public class Program
{
    public static void Main()
    {
        Dog dog = new Dog();
    }
}

Output will be

In Animal's constructor
In Dog's constructor

View Demo

Call parent's constructor explicitly.

In the above examples, our Dog class constructor calls the default constructor of the Animal class. If you want, you can specify which constructor should be called: it is possible to call any constructor which is defined in the parent class.

Consider we have these two classes.

class Animal
{
    protected string name;

    public Animal()
    {
        Console.WriteLine("Animal's default constructor");
    }    

    public Animal(string name)
    {
        this.name = name;
        Console.WriteLine("Animal's constructor with 1 parameter");
        Console.WriteLine(this.name);
    }
}

class Dog : Animal
{
    public Dog() : base()
    {
        Console.WriteLine("Dog's default constructor");
    }  

    public Dog(string name) : base(name)
    {
        Console.WriteLine("Dog's constructor with 1 parameter");
        Console.WriteLine(this.name);
    }
}

What is going here?

We have 2 constructors in each class.

What does base mean?

base is a reference to the parent class. In our case, when we create an instance of Dog class like this

Dog dog = new Dog();

The runtime first calls the Dog(), which is the parameterless constructor. But its body doesn't work immediately. After the parentheses of the constructor we have a such call: base(), which means that when we call the default Dog constructor, it will in turn call the parent's default constructor. After the parent's constructor runs, it will return and then, finally, run the Dog() constructor body.

So output will be like this:

Animal's default constructor
Dog's default constructor

View Demo

Now what if we call the Dog's constructor with a parameter?

Dog dog = new Dog("Rex");

You know that members in the parent class which are not private are inherited by the child class, meaning that Dog will also have the name field.
In this case we passed an argument to our constructor. It in his turn passes the argument to the parent class' constructor with a parameter, which initializes the name field.

Output will be

Animal's constructor with 1 parameter
Rex
Dog's constructor with 1 parameter
Rex

Summary:

Every object creation starts from the base class. In the inheritance, the classes which are in the hierarchy are chained. As all classes derive from Object, the first constructor to be called when any object is created is the Object class constructor; Then the next constructor in the chain is called and only after all of them are called the object is created

base keyword

  1. The base keyword is used to access members of the base class from within a derived class:
  2. Call a method on the base class that has been overridden by another method. Specify which base-class constructor should be called when creating instances of the derived class.