Reading From a Text File in C#

I have the following program that will send (output) information to a text file, but now I want to read (input) from the text file. Any suggestions would be greatly appreciated. I have commented out a couple of things that "I think" I need to do; but I am not really certain how to proceed.

using System.Windows.Forms;
using System.IO;

namespace Input_Output
{
    public partial class Grades : Form
    {
        private StreamWriter output;

        private StreamReader input;


        public Grades()
        {
            InitializeComponent();
        }

        private void label4_Click(object sender, EventArgs e)
        {

        }

        private void btnCreate_Click(object sender, EventArgs e)
        {
            btnEnter.Visible = true;
            btnClose.Visible = true;
            txtFirst.Visible = true;
            txtLast.Visible = true;
            lblFirst.Visible = true;
            lblLast.Visible = true;
            listBox1.Visible = true;
            lblStatus.Visible = true;
            lblGrade.Visible = true;
            lblCourse.Visible = true;
            cmbID.Visible = true;
            lblID.Visible = true;
            txtCourse.Visible = true;
            txtGrade.Visible = true;
            lblStatus.Visible = true;

            DialogResult result;
            string fileName;
            using (SaveFileDialog chooser = new SaveFileDialog())
            {
                result = chooser.ShowDialog();
                fileName = chooser.FileName;
            }
            output = new StreamWriter(fileName);
            btnCreate.Enabled = false;
            txtFirst.Visible = true;
            txtLast.Visible = true;
            lblFirst.Visible = true;
            lblLast.Visible = true;
            btnEnter.Visible = true;
            btnClose.Visible = true;
        }


        private void btnClose_Click(object sender, EventArgs e)
        {
            //Close button pushes information from the listbox in to the text file

            output.Close();
            lblStatus.ForeColor = Color.Red;
            lblStatus.Text = "Output File";
            btnCreate.Enabled = true;
            listBox1.Items.Clear();
            cmbID.Text = "";
        }

        private void btnEnter_Click(object sender, EventArgs e)
        {
            // Enter button sends information to the list box, a Message Box prompts user to check for accuracy.  
            //Close button pushes information to the Text file.
            string last = "";
            string first = "";
            string course = "";
            string grade = "";

            if (txtFirst.Text != "" && txtLast.Text != "" && txtCourse.Text != "")
            {
                last = txtFirst.Text;
                first = txtLast.Text;
                course = txtCourse.Text;
                grade = txtGrade.Text;
                output.WriteLine (last + "\t"+ "\t" + first + ":"+ "\t" + cmbID.SelectedItem + "_" + course + "_" + grade );

                listBox1.Items.Add(txtLast.Text + "," + txtFirst.Text + ":" + cmbID.SelectedItem + "-" + txtCourse.Text + "-" + txtGrade.Text);
                lblStatus.ForeColor = Color.Navy;
                lblStatus.Text = "Entry Saved";
                txtFirst.Text = "";
                txtLast.Text = "";
                txtCourse.Text = "";
                txtGrade.Text = "";
                txtFirst.Focus();
            }
            else
            {     
                lblStatus.ForeColor = Color.Red;
                lblStatus.Text = "Empty text box or boxes";
            }
            MessageBox.Show("Please verify that the information is correct before proceeding");
        }

        private void btnExit_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void Grades_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            DialogResult result;
            string fileName;
            using (OpenFileDialog chooser = new OpenFileDialog())
            {
                result = chooser.ShowDialog();
                fileName = chooser.FileName;
            }
            //while loop?
            //if variable is null, it's the end of the record
            //variable= !null 
            //txt read int variable TxtFile.Text += Rec + "\r\n";   while rec !=null;
        }
    }
}

Solution 1:

To read a text file one line at a time you can do like this:

using System.IO;

using (var reader = new StreamReader(fileName))
{
    string line;
    while ((line = reader.ReadLine()) != null)
    {
        // Do stuff with your line here, it will be called for each 
        // line of text in your file.
    }
}

There are other ways as well. For example, if the file isn't too big and you just want everything read to a single string, you can use File.ReadAllText()

myTextBox.Text = File.ReadAllText(fileName);

Solution 2:

It's just one line of code:

string content = System.IO.File.ReadAllText(@"C:\textfile.txt");

Solution 3:

Try this:

if(result == DialogResult.OK && fileName != null)
{
    try
    {
        var fileText=File.ReadAllText(fileName);
    }
    catch(Exception ex)
    {
        //Handle exception here
    }
}

It will read all the data from the selected file into the fileText variable.