calculating the derivative

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C#

i need to calculate the derivative of each and every pixel ,store in an array and display on a text file
im new to c#.net..
i calculated the derivate of each pixel..and stored in an array... now i need to display on a text file.. i couldnt knw how to display on a text file....
if there are any mistakes in my code please correct me an d help me to display the pixel values on a text file...

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;

namespace derivative
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        Bitmap newbitmap;
        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog open = new OpenFileDialog();
            open.Filter = "Image Files(*.jpg; *.jpeg; *.gif; *.bmp)|*.jpg; *.jpeg; *.gif; *.bmp";
            if (open.ShowDialog() == DialogResult.OK)
            {
                pictureBox1.ImageLocation = open.FileName;
                newbitmap = new Bitmap(open.FileName);
                //opened = true;
                /* System.Drawing.Image img = System.Drawing.Image.FromFile( open.FileName     );
                 MessageBox.Show("Width: " + img.Width + ", Height: " + img.Height);*/
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Color n = new Color();
            int x;
            int y;
            int []s;
            s = new int[1000];
            int a;
            for (a = 0; a < s.Length;a++)
            {
                 for (x = 1; x <= newbitmap.Width - 1; x++)
             
                {

                    for (y = 1; y <= newbitmap.Height - 1; y++)
                  
                    {
                       


                    

//derivative of each pixel
                      
                        s[a] = (int)((newbitmap.GetPixel(x + 1, y).R) + (newbitmap.GetPixel(x - 1, y).R) + (newbitmap.GetPixel(x, y + 1).R) + (newbitmap.GetPixel(x, y - 1).R) - (newbitmap.GetPixel(x, y).R * 4));

                       

                    }


                }



            }



        }
    }
}

im not sure if this is the correct way to store in an array or not...plz help mee.....