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A circular queue (a ring buffer) is a kind of queue that forms a circle, internally uses an array, as well as the maximum number of elements that can be placed inside the queue is limited. You need to specify two variables that indicate indices of the front and rear elements. The front one points to the element that will be dequeued first. The rear one points to the element that is the last in the queue.

Let's start with the implementation:

public class CircularQueue(int size)
    where T : struct
{
    private readonly T[] _items = new T[size];
    private int _front = -1;
    private int _rear = -1;
    private int _count = 0;
    public int Count { get { return _count; } }

    public bool Enqueue(T item)
    {
        if (_count == _items.Length) { return false; }

        if (_front < 0) { _front = _rear = 0; }
        else { _rear = ++_rear % _items.Length; }

        _items[_rear] = item;
        _count++;
        return true;
    }

    public T? Dequeue()
    {
        if (_count == 0) { return null; }

        T result = _items[_front];
        if (_front == _rear) { _front = _rear = -1; }
        else { _front = ++_front % _items.Length; }

        _count--;
        return result;
    }

    public T? Peek()
    {
        if (_count == 0) { return null; }
        return _items[_front];
    }
}

You can see how it works with the following code:

CircularQueue queue = new(8);
queue.Enqueue(2);
queue.Enqueue(-4);
queue.Enqueue(1);
queue.Enqueue(8);
queue.Enqueue(5);
int item = queue.Dequeue();
Console.WriteLine(item);

This entry can contain content, including parts of code, from one of my books, namely from the Second Edition of C# Data Structures and Algorithms, published by Packt Publishing in 2024. If you want to learn much more about a circular queue, as well as about other queues and other data structures in general, I encourage you to take a look at the mentioned book. You can buy it here.

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