Inheriting from List in C# – How to Guide

c++collectionsinheritancelist

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.

Best Answer

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.

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No - don't use new here; that doesn't give you polymorphism. List<T> isn't intended for inheritance in this way; use Collection<T> and override the ~~Add~~ InsertItem method.

public class LimitedCollection<T> : Collection<T>
{
    private int _maxitems = 500;

    protected override void InsertItem(int index, T item)
    {
        base.InsertItem(index, item);
        TrimData(); /* Delete old data if lenght too long */
    }

    private void TrimData()
    {
        int num = Math.Max(0, base.Count - _maxitems);
        while (num > 0)
        {
            base.RemoveAt(0);
            num--;
        }
    }
}

Best Answer

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