C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections;

namespace Construction_Survey_Layout
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        // PN is the index. I didn't set it to 0 because I want it to retain it's value
        // It is at 0 the first time run.
        public int PN;

        private void btnNew_Click(object sender, EventArgs e)
        {
            ArrayList Projects = new ArrayList();            
            // The error was directed at PN here.
            Projects[PN] = textBoxAddNew.ToString();

            Projects.Add(Projects[PN]);
                                    
            textBoxAddNew.Clear();

            Projects.Sort();

            for(int ctr = 0; ctr <= PN; ctr++)
            {
                listBox1.Items.Add(Projects[ctr]);                
            }
            PN++;
            
            // The following is the error message I got after entering a Project Name in the text box
            // and clicked the Add New button:
            //Index was out of range. Must be non-negative and less than the size of the collection.
            //  Parameter name: index
        }

        
    }
}