CircularBuffer.cs

public class CircularBuffer<T>
{
    private readonly int _size;
    private readonly T[] _items;

    private int _writeLocation = -1;

    private readonly object _lock = new object();

    public CircularBuffer(int size)
    {
        _size = size;
        _items = new T[size];
    }

    public void Add(T item)
    {
        lock (_lock)
        {
            _writeLocation++;
            _writeLocation %= _size;
            _items[_writeLocation] = item;
        }
    }
}