A parent class needs to know his child classes

Question

Probem: A parent abstract class must be aware of all his existing child classes. The parent class must also be able to list all the class names of his child classes through a method.

Context: In my situation, the parent class is used to represent any input data type for a web form (such as email, text, boolean, integer, image, and so on). The child classes represent each specific data types. So there is a Boolean class, an Image class, an Integer class, etc.

The language I'm using is C#.

How would you do that?

edit: The reason why I need to know the child classes is because I need to know all the input data types available in order to list them. Also, I need to access the methods of each data type (child class) to get its properties.

So, for example, I need to get an instance of the Image class simply with the string "Image" that is stored in a database. If the base class knowns all its children, I will be able to ask it to get me the child represented by the string "Image", and it will return an instance of the Image class.

I thought of implementing it by making each child a Singleton, and adding itself to a list of children (at construction) that would be a private field of the base DataType class.

Answer 1

You can check out this post. Looks like you need to iterate through known types and see if they are a subclass of the parent class using Type.IsSubclassOf

Answer 2

You can write in the constructor code that will notify some static class that something new was created.

Something like:

class A {
 A() {
  myBuilder.notifyMe(this);
 }
}

myBuilder is a static class with code:

void notifyMe(Object o) {
 // do whatever you want here
 // maybe something like
 someList.Add(typeof(o).Name)
}

edit: Ok, this solution, of course, requires that the subclasses are instantiated first. If you want to know all the subclasses when program starts I think the only way is to scan entire class path and check for each class is it subclass of wanted one.