从具有唯一代码的文本文件C＃中读取特定行，该代码将显示行号

Question

I want to read a specific line from a textfile in C# language with a unique code that will display the line number, And I am struggling to get this on any website, They show a lot but not exactly what I am looking for, This is my code:-

namespace MyQuestion2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnDisplay_Click(object sender, EventArgs e)
        {
            //opens the file dialog
            OpenFileDialog ofd = new OpenFileDialog();
            //checks to see if the user does open a file dialog
            if (ofd.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                //reads the file that the user opens
                StreamReader sr = new StreamReader(File.OpenRead(ofd.FileName));
                txtOutput.Text = sr.ReadToEnd();
                sr.Dispose();
            }
        }

        private void TxtProductCode_TextChanged(object sender, CancelEventArgs e)
        {
            TextBox tb = sender as TextBox;
            if (tb != null)
            {
                int i;
                if (int.TryParse(tb.Text, out i))
                {
                    if (i >= 0 && i <= 5)
                        return;
                }
            }
            MessageBox.Show("The number you have put is less than 0 or higher that 5! Please try again");
            e.Cancel = true;
        }
    }
}

Thanks in advance.