reference the "self" type in a static method - C#

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't, basically. If a call to a static member which is only declared in a base class is actually expressed in terms of the derived class, like this:

// Urgh
Encoding ascii = ASCIIEncoding.ASCII;

// Worse yet (and yes, I've seen this)
Encoding ascii = UTF8Encoding.ASCII;

then the compiler silently transforms that to:

Encoding ascii = Encoding.ASCII;

The fact that the original source contained the name of a derived class is not preserved in the compiled code, so it can't be acted on at execution time.

Answer 2

How about using generics to provide the type to produce:

public static T getInstance<T>() where T : A, new()
{
    return new T();
}