dataItem in dataSet, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            DataSet dataSet = new DataSet();
            for (int i = 0; i < dataSet.SIZE; i++)
            {
                DataItem dataItem = new DataItem();            
                Build(dataItem, dataItem.SIZE);
                dataSet.Add(dataItem);
            }
            
            dataSet.ToPrint();
        }
        
        private static void Build(DataItem dataItem, int size)
        {
            for (int i = 0; i < size; i++) 
            {
                dataItem.Add(new Random().NextDouble());
                System.Threading.Thread.Sleep(10);
            }
        }
    }
    
    public class DataItem
    {
        public readonly int SIZE = 10;
        private int index = 0;
        private double[] values;
        
        public DataItem()
        {
            this.Initial();
        }
        
        private void Initial()
        {
            values = new double[SIZE];
        }
        
        public void Add(double value)
        {
            if (index < SIZE)
            {
                this.values[index++] = value;
            }
        }
        
        public void ToPrint()
        {
            foreach(double value in this.values)
            {
                Console.Write("{0} ", value);
            }
            Console.WriteLine();
        }       
    }
    
    public class DataSet
    {
        public readonly int SIZE = 10;
        public int index = 0;
        private DataItem[] dataItems;
        
        public DataSet()
        {
            this.Initial();
        }
        
        private void Initial()
        {
            this.dataItems = new DataItem[SIZE];
            for (int i = 0; i < SIZE; i++)
            {
                this.dataItems[i] = new DataItem();   //// Are you forgetting initialising DataItem in DataSet?
            }
        }
        
        public void Add(DataItem dataItem)
        {
            if (index < SIZE) 
            {
                this.dataItems[index++] = dataItem;
            }
        }
        
        public void ToPrint()
        {
            foreach (var dataItem in this.dataItems)
            {
                dataItem.ToPrint();
            }
        }
    }
}

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