C#_OOP_Features_Manager<T>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace App1b
{
    class Manager<T>
    {
        private const int SIZE = 5;
        private T[] entities;
        private int counter = 0;

        public Manager()
            : this(SIZE)
        { }

        public Manager(int size)
        {
            this.entities = new T[size];
        }

        public void AddEntity(T entity)
        {
            if (counter >= entities.Length) {
                throw new InvalidOperationException("The stack is full");
            }
            entities[counter] = entity;
            counter++;
        }

        public T RemoveEntity()
        {
            if (counter == 0) {
                throw new InvalidOperationException("The stack is empty");
            }
            T temp = entities[counter - 1];
            entities[counter - 1] = default(T);
            counter--;
            return temp;
        }

        public int GetNumberOfEntities()
        {
            return counter;
        }
    }
}