Question

Here is the code as it now stands, I will include all of the code of the program as I left some bits out before. The bits I have changed due to your help I have emphasized with asterisks and ///
The first class is the standard one created from Windows Forms when directly editing your form.

namespace DistanceEstimatorFinal
{
    public partial class Form1 : Form
    {
        private bool saved;

        public Form1()
        {

            dataPoints mydataPoints = new dataPoints();
            InitializeComponent();
            dataPoint a = mydataPoints.getItem(0);
            latTextBox.Text = a.CurLatitude;
            longTextbox.Text = a.CurLongtitude;
            eleTextBox.Text = a.CurElevation;
            saved = true;

        }             

        private void latTextBox_TextChanged(object sender, EventArgs e)
        {

        }

        private void openDataListToolStripMenuItem_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Filter = "CSV files (*.csv)|*.csv|Text files ( *.txt)|*.txt |All files (*.*)|*.*";
            if (ofd.ShowDialog(this).Equals(DialogResult.OK))
            {
                *var dp = new dataPoints (ofd.FileName);* /////

            }
        }       



        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (saved)
            {
                if (MessageBox.Show("Save?", "Data Not Saved", MessageBoxButtons.YesNo).Equals(DialogResult.Yes))
                {
                    SaveFileDialog sfd = new SaveFileDialog();
                    sfd.ShowDialog();

                }

            }
        }

        private void saveAsToolStripMenuItem_Click(object sender, EventArgs e)
        {
            SaveFileDialog sfd1 = new SaveFileDialog();
            sfd1.Filter = "CSV files (*.csv)|*.csv|Text files ( *.txt)|*.txt |All files (*.*)|*.*";
            sfd1.ShowDialog();
        }

    }
}     

This class was designed to read in the data from a file, I am currently adapting it to read in a file from the open function.

namespace DistanceEstimatorFinal
{    
    public class dataPoints
    {              
        List<dataPoint> Points;
        string p;

        public dataPoints(string path)
        {
            p = path;
            Points = new List<dataPoint>();

            StreamReader tr = new StreamReader(p);

            string input;
            while ((input = tr.ReadLine()) != null)
            {
                string[] bits = input.Split(',');
                dataPoint a = new dataPoint(bits[0],bits[1],bits[2]);              
                Points.Add(a);  


            }

            tr.Close();
        }





        internal dataPoint getItem(int p)
        {
            if (p < Points.Count)
            {
                return Points[p];
            }
            else
                return null;
        }
    }

}

This file held the three variables Distance, latitude and Longtitude.

namespace DistanceEstimatorFinal
{
    class dataPoint
    {
        private string latitude;
        private string longtitude;
        private string elevation;

        public dataPoint()                               //Overloaded incase no value available
        {
            latitude = "No Latitude Specified";
            longtitude = "No Longtitude Specified";
            elevation = "No Elevation Specified";

        }

        public dataPoint(string Latitude, string Longtitude, string Elevation)
        {

            // TODO: Complete member initialization
            this.latitude = Latitude;
            this.longtitude = Longtitude;
            this.elevation = Elevation;

        }

        public string CurLongtitude { get { return this.longtitude; } }
        public string CurLatitude { get { return this.latitude; } }
        public string CurElevation { get { return this.elevation; } }

    }

Answer 1

Your pathFile is a method local variable, so it’s inacccesible anywhere except the body of that method (here openDataListToolStripMenuItem_Click).

You could add a parameter to your dataPoints constructor to pass that value to the class:

public class dataPoints
{
    List<dataPoint> Points;
    public dataPoints(string path)
    {
        Points = new List<dataPoint>();
        //here `path` from constructor arguments
        TextReader tr = new StreamReader(path); 
        //...rest part of your code
    }

Besides you’ll have to pass the value to this constructor. You didn’t show the code, you have to create dataPoints instanses.

var dp = new dataPoints(pathFile);

But remember, pathFile is accessible only in openDataListToolStripMenuItem_Click. So you should either create the dataPoints right there, or make your pathFile a field of a form for it to be accessible in any method of that form. Then you’d get an opportunity to access pathFile in any method of this form.

---

According to your previous post, this should look like:

private void openDataListToolStripMenuItem_Click(object sender, EventArgs e)
{
    OpenFileDialog ofd = new OpenFileDialog();
    ofd.Filter = "CSV files (*.csv)|*.csv|Text files ( *.txt)|*.txt |All files (*.*)|*.*";
    if (ofd.ShowDialog(this).Equals(DialogResult.OK))
    {            
        //actually you don't even need to have a separate `pathFile` variable
        //just pass the value from the dialog straight to your `dataPoints` object
        var dp = new dataPoints(ofd.FileName);
        //...rest of your code
    }
}

P.S.: off-topic, but, please, consider reading MSDN Guidelines for Names