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Why Can't We Instantiate Abstract Class

A few days back my colleague asked me a question – Why cannot we instantiate an abstract class?. I found something on the internet which I would like to share with you.

The answer to the above question is that we cannot instantiate abstract class because by default abstract class contains a protected constructor. It means you can call the constructor of an abstract class when we derive the abstract class and call the constructor of the derived class.

Let us work on an example. In this example, we will create an abstract class, and inside the class, we will create a protected constructor. Although you can also create a public constructor and the code does not give the compilation error still the compiler considers it as a protected constructor and therefore does not allow you to create an instance.

Example

Consider the following abstract class.

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

Create a new class derived from an abstract class.

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

In the above code, we are calling the constructor of an abstract class using the base keyword. Now create the instance of the derived class.

DerivedClass cls = new DerivedClass(10);

Output

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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