How do I write a program that reads a file and then counts each time a character appears

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


namespace Simple_File_Demo
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        

        private void button1_Click(object sender, EventArgs e)
        {
            listBox1.Items.Clear();
            string line;
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.DefaultExt = "txt";
            ofd.ShowDialog();

            StreamReader sr = new StreamReader(ofd.FileName);

            while (sr.Peek() > 0)
            {
                line = sr.ReadLine();
                listBox1.Items.Add(line);
            }
            sr.Close();
        }

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

//David Crandall
//October 22nd, 2015
//This program counts the letters for each inputed phrase

namespace Letter_Counter
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnCount_Click(object sender, EventArgs e)
        {

            lstbxCounted.Items.Clear();

            //establishes variables that will be used later
            string myData = txtPhrase.Text.ToUpper();
            string Letters = "";
            string temp;
            int Count = 0;

            //This array will allow the counting of letters
            int [] intz = new int[26];

            //Removes everything that is a non-letters
            for (int i = 0; i < 26; i++)
            {
                intz[i] = 0;
            }
            
            for (int i = 0; i < myData.Length; i++)
            {
                temp = myData.Substring(i, 1);
                if (temp.CompareTo("A") >= 0 && temp.CompareTo("Z") <= 0)
                {
                    Letters = Letters + temp;
                }
            }

            //For each of the 
            for (int l = 0; l < 26; l++)
            {
                Count = 0;
                for (int i = 0; i < Letters.Length; i++)
                {
                    string sub = Letters.Substring(i, 1);
                    int subz = char.Parse(sub) - 'A';
                    if(subz == l)
                    {
                        Count++;
                    }
                }
                intz[l] = Count;

                //Outputs the counted letters to listbox
                lstbxCounted.Items.Add(Convert.ToChar(l + 'A') + "\t" + intz[l]);
            }
        }
    }
}