Inheriting from List<T>

46
votes

What is the fastest way to implement a new class that inherits from List<T>?

class Animal {}

class Animals : List<Animal> {} // (1)

One problem I've encountered: By simply doing (1), I've found that I'm not getting the benefit of inheriting any constructors from List<T>.

In the end, I'd like Animals to behave a lot like a List<T> (e.g., can be constructed, compatibility with Linq). But in addition, I'd also like to be able to add my own custom methods.

6
c#listinheritancecollections
What you've posted looks pretty good to me! – Stuart
Your code above would do it. You would need to implement any constructors and just call through to the relevant constructor on the base call (List<Animal>). You can create any custom methods within the Animals class – detaylor
possible duplicate of C#: Inheritance Problem with List<T> – George Stocker

6 Answers

51
votes

If you want to create a publicly exposed animal collection you should not inherit from List<T> and instead inherit from Collection<T> and use the postfix Collection in the class name. Example: AnimalCollection : Collection<Animal>.

This is supported by the framework design guidelines, more specifically:

DO NOT use ArrayList, List<T>, Hashtable, or Dictionary<K,V> in public APIs. Use Collection<T>, ReadOnlyCollection<T>, KeyedCollection<K,T>, or CollectionBase subtypes instead. Note that the generic collections are only supported in the Framework version 2.0 and above.

29
votes

Constructors are not inherited along with the class. You have to reimplement your desired constructors.

public class AnimalsCollection : List<Animal>
{
     public AnimalsCollection(IEnumerable<Animal> animals) : base(animals) {} 
}
13
votes

Deriving from List<T> is not advised. Primarily because List was never meant for extension, but for performance.

If you want to create your own specific collection, you should inherit from Collection<T>. In your case it would be:

class Animals : Collection<Animal> {}
6
votes

Bear in mind that inheriting from List isn't as fully featured as you may need, a lot of the members are not virtual so the only way of covering the base implementation is to shadow it with the new syntax (as opposed to override).

If you need to start exposing custom behaviour on standard list actions, I would implement all the list interfaces on a type that simply uses an inner list for the actual storage.

This is heavily dependent on your final requirements.

4
votes
class Animals : List<Animal> {}

seemes the best way, because you can use it just after defining it like this.

So my advice is your questions title: Inherit List<T> ;-)

4
votes

It isn't necessary to inherit from List to use collection initialization syntax or use Linq extension methods.

Just implement IEnumerable and also an Add method.