FixedSizeLinkedList.cs

public class FixedSizedLinkedList<T> : LinkedList<T>
{
    private readonly object syncObject = new object();

    public int Size { get; private set; }

    public FixedSizedLinkedList(int size)
    {
        Size = size;
    }

    public new void AddFirst(T obj)
    {
        base.AddFirst(obj);
        lock (syncObject)
        {
            while (base.Count > Size)
            {
                base.RemoveLast();
            }
        }
    }

    public new void AddLast (T obj)
    {
        base.AddLast(obj);
        lock (syncObject)
        {
            while (base.Count > Size)
            {
                base.RemoveFirst();
            }
        }
    }
}