Question

Is it possible to create an abstract method that must return an instance of the derived class? I can do this:

abstract class Base
{
   public abstract Base GetObj();
}

class Derived : Base
{
   public Derived() { }

   public override Base GetObj()
   {
       return new Derived();
   }
}

But I was wondering if there was a way to do it such that Derived::GetObj() is forced to return a Derived?

Thanks.

Answer 1

Using generics should make this possible:

abstract class Base<T>
    where T : Base<T>
{
   public abstract T GetObj();
}

class Derived : Base <Derived>
{
   public Derived() { }

   public override Derived GetObj()
   {
       return new Derived();
   }
}

You could even simplify this even more (if all of the derived instances are created with default constructors):

abstract class Base<T>
    where T : Base<T>, new()
{
    public static T GetObj()
    {
        return new T();
    }
}

class Derived : Base<Derived>
{
    public Derived() { }
}