Creating lines of a text file into an array?

[tinfanide]

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
 
namespace CompareRegistryFiles
{
    class Program
    {
        static void Main(string[] args)
        {
            string sDesktopPath = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
            string sFilePath = sDesktopPath + "\\" + "3.txt";
            using (StreamReader oStreamReader = new StreamReader(sFilePath))
            {
                string line;
                string[] arr = new String[3];
                int x = 0;
                while ((line = oStreamReader.ReadLine()) != null)
                {
                    // 3.txt
                    // a
                    // b
                    // [space]
                    // d
 
                    Console.WriteLine(x + " " + line);
 
                    // if string[] arr = new String[3]
                    // error: Index was outside the bounds of the array.
                    arr[x] = line;
                    x++;
                }
                Console.ReadLine();
            }
        }
    }
}

Please see the comments above.
If
if string[] arr = new String[4],
no error.
But there is only 4 lines and the arr index x starts from 0. So the size of arr should be no more than 3.
Could anyone explain to me why the error occurs?
Thanks.