c# wpf edit one line in textbox

using System;
using System.Text.RegularExpressions;

public class Test
{

  public bool AddToTexBoxOfLine(TextBox box, int lineNumber, string text)
        {
            if (lineNumber <= 0)
                throw new Exception("The number must be 1 or larger\nNumbers smaller than 1 are not acceptable");
            string textbox_text = box.Text;
            //var split = textbox_text.Split(new string[] { Environment.NewLine }, StringSplitOptions.None);
            var split = Regex.Split(textbox_text, "\r\n|\n\r|\r|\n|"+ Environment.NewLine);
            int countElement = (lineNumber > split.Length ? lineNumber : split.Length);
            string[] newColeccion = new string[countElement];

            //no move location
            lineNumber = lineNumber - 1;

            for (int i = 0; i < lineNumber; i++) newColeccion[i] = "";
            for (int i = 0; i < split.Length; i++) newColeccion[i] = split[i];
            for (int i = 0; i <= newColeccion.Length; i++)
            {
                if(i == lineNumber)
                {
                    newColeccion[i] = text;
                    break;
                }
            }
            box.Text = string.Join(Environment.NewLine, newColeccion);
            return true;
        }
}