How to overwrite list?

Question

I need collection that is similar to list, instead of always adding items when particular length is reached it should start overwriting values from the first index and continue in circular order.

I.e. for 4 items limit:

specialList.Add(100); // {100}
specialList.Add(101); // {100, 101}
specialList.Add(102); // {100, 101, 102}
specialList.Add(103); // {100, 101, 102, 103}
specialList.Add(104); // {104, 101, 102, 103}
specialList.Add(105); // {104, 105, 102, 103}

Answer 1

(Updated to show a General list class) This is a class which could be used for special list class that loops (cycles to the first item) when reached to last element:

public class ListCycle<T> : IList<T>
{

    int curIndex = -1;
    List<T> list;
    int nMax;

    public ListCycle(int n)
    {
        list = new List<T>(n);
        nMax = n;
    }

    /// <summary>returns the current index we are in the list</summary>
    public int CurIndex { get { return curIndex; } }

    public int IndexOf(T item) { return list.IndexOf(item); }
    public bool Contains(T item) { return list.Contains(item); }
    public int Count { get { return list.Count; } }
    public bool IsReadOnly { get { return false; } }
    public IEnumerator<T> GetEnumerator() { return list.GetEnumerator(); }
    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() { return list.GetEnumerator(); }

    public T this[int index]
    {
        get { return list[index]; }
        set { list[index] = value; }
    }

    public void Add(T item)
    {
        curIndex++; if (curIndex >= nMax) curIndex = 0;
        if (curIndex < list.Count)
            list[curIndex] = item;
        else
            list.Add(item);
    }

    public void Clear()
    {
        list.Clear();
        curIndex = -1;
    }

    //other mehods/properties for IList ...
    public void Insert(int index, T item) { throw new NotImplementedException(); }
    public bool Remove(T item) { throw new NotImplementedException(); }
    public void RemoveAt(int index) { throw new NotImplementedException(); }
    public void CopyTo(T[] array, int arrayIndex) { throw new NotImplementedException(); }

}

Usage is easy as:

var list = new ListCycle<int>(10);

//fill the list
for (int i = 0; i < 10; i++)
{
    list.Add(i);
}

//now list is:
// 0, 1, 2, 3, ...

//add more items will start from first
list.Add(100); //overrides first item
list.Add(101); //overrides second item

//now list is:
// 100, 101, 2, 3, ...