Question

I am trying to create a static method that would return an instance of the class, something like:

class A {
   public static A getInstance() {
      return new A();
   }
}

The problem I am having is that if I have a subclass B derived from A, I would like B.getInstance() to return an instance of B, and not A. In PHP world, you could use a keyword “self” to reference to your own type, so your getInstance() would look like:

public static function getInstance() {
   return new self();
}

What’s the best way to go about this?

Answer 1

You can hide and so in a way override the getInstance method in the inheriting type like this:

class B : A {
    public static new B getInstance() {
        return new B();
    }
}

The new keyword specifies that the hiding of the inherited method was intentional, otherwise you get a compiler warning.

Complete example:

using System;

class A {
    public static A getInstance() { return new A(); }
    public void PrintSelf() {Console.WriteLine(this.GetType().Name);}
}

class B : A {   
    public static new B getInstance() {
        return new B();
    }
}

public class MyClass {
    public static void Main() {
        A a = A.getInstance();
        B b = B.getInstance();
        a.PrintSelf();
        b.PrintSelf();
    }
}