How do I override List's Add method in C#?

Question

I am currently looking to make my own collection, which would be just like a regular list, except that it would only hold 10 items. If an item was added when there were alre

Answer 1

Your description of your requirement sounds like a Circular Buffer.

I implemented my own - similar to this implementation on CodePlex except that mine implements IList<T>.

Some of the other answers suggest using a Queue<T> - but this isn't quite the same thing, as it only allows FIFO access.

As a general point, it's not recommended to derive from List<T> - instead derive from Collection<T>, and implement any additional stuff you need. But for a circular buffer it's probably more appropriate to use a private array rather than deriving from Collection<T> like the CodePlex implementation.

Answer 2

You could extend System.Collections.ObjectModel.Collection and override the InsertItem method to get the behaviour you want, and it also implements IList

Answer 3

It seems like the best I can do is this:

class MostRecentList<T> : System.Collections.Generic.List<T> {
        private int capacity;

        public MostRecentList(int capacity) : base() {
            this.capacity = capacity;
        }

        public new void Add(T item) {
            if (base.Count == capacity) {
                base.RemoveAt(0);
            }
            base.Add(item);
        }
}

Since the add() method is not marked as virtual.

Answer 4

You can also implement the add method via

public new void Add(...)

in your derived class to hide the existing add and introduce your functionality.

Edit: Rough Outline...

class MyHappyList<T> : List<T>
{
    public new void Add(T item)
    {
        if (Count > 9)
        {
            Remove(this[0]);
        }

        base.Add(item);
    }
}

Just a note, figured it was implied but you must always reference your custom list by the actual type and never by the base type/interface as the hiding method is only available to your type and further derived types.

Answer 5

You could take a look at the C5 collection library. They have an ArrayList<T> that implements IList<T> and have a virtual Add method. The C5 collection library is an awesome collection of lists, queues, stacks etc... You can find the C5 library here:

http://www.itu.dk/research/c5/

Answer 6

You can't override Add(), it is not a virtual method. Derive from IList instead and use a private Queue member for the implementation.