How to get a collection variable of base type items to hold collection instances of derived type items?

The problem is that the two lists aren't covariant:

Quote:

In object-oriented programming, a covariant return type of a method is one that can be replaced by a "narrower" type when the method is overridden in a subclass. A notable language in which this is a fairly common paradigm is C++. C# does not support return type covariance.

(Covariant return type - Wikipedia[^])

What that basically means is that if you construct collections:

class Animal {...}
class Ape : Animal {...}
class Feline : Animal {...}

List<Ape> apes = new List<Ape>();
List<Feline> cats = new List<Feline>();
List<Animal> animals = new List<Animal>();

Then because an Ape is an Animal, you can add a new species to either collection:

Ape newSpecies = new Ape("Pans Sapiens");
apes.Add(newSpecies);
animals.Add(newSpecies);

But if you try to add it to the Felines:

cats.Add(newSpecies);

You will get a compilation error:

Argument 1: cannot convert from 'GeneralTesting.frmMain.Ape' to 'GeneralTesting.frmMain.Feline'

And that is what you would expect.
But ... if you could do what you want, you could do this:

Ape newSpecies = new Ape("Pans Sapiens");
animals = cats;
animals.Add(newSpecies);

And you would have an Ape in your Feline collection - which means your application will crash later on when you try to use it.

You can add all the items:

animals = new List<Animal>();
List<Ape> apes = new List<Ape>();
List<Feline> cats = new List<Feline>();
...
animals.AddRange(apes);
animals.AddRange(cats);

Or even explicitly:

animals = new List<Animal>();
List<Ape> apes = new List<Ape>();
List<Feline> cats = new List<Feline>();
...
animals.AddRange(apes.ConvertAll(x => (Animal)x));
animals.AddRange(cats.ConvertAll(x => (Animal)x));

But you can't assign items which aren't covariant.