Articles → .NET → Why Can't We Instantiate Abstract Class

Why Can't We Instantiate Abstract Class

Few days back my colleague asked me a question – ‘Why can’t we instantiate abstract class’. I found something on internet which I would like to share it with you.

The answer of the above question is – ‘we can’t instantiate abstract class because by default abstract class contains protected constructor’. What does that mean? The answer is - it means you can call the constructor of an abstract class when we derive the abstract class and call the constructor of derived class.

Let us work on an example. In this example I will create an abstract class and inside the class I will create a protected constructor. Although you can also create a public constructor and the code doesn’t give the compilation error but still the compiler think of it as a protected constructor and therefore not allow you to create an instance.

So here is a sample abstract class for you

public abstract class AbstractClass {
  protected AbstractClass(int x) {
    Console.WriteLine("Abstract Constructor");
  }
}

Now I will create a new class which is derived from an abstract class

public class DerivedClass: AbstractClass {
  public DerivedClass(int x): base(x) {
    Console.WriteLine(x);
  }
}

So here we are calling the constructor of an abstract class using ‘base’ keyword. Now create the instance of derived class

DerivedClass cls = new DerivedClass(10);

Now when you run the program you could see that first abstract class constructor will be called and then derived class constructor will be called.

Picture showing the constructor invocation of abstract class constructor followed by derived class constructor

Click to Enlarge

Posted By -	Karan Gupta

Posted On -	Sunday, January 11, 2015

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