Loading text file into listbox

This will work

List<string> lines = new List<string>();
using (StreamReader r = new StreamReader(f))
{
    string line;
    while ((line = r.ReadLine()) != null)
    {
        lines.Add(line);
    }
}

  OpenFileDialog f = new OpenFileDialog();
    if (f.ShowDialog() ==DialogResult.OK)
    {
        listBox1.Items.Clear();

        List<string> lines = new List<string>();
        using (StreamReader r = new StreamReader(f.OpenFile()))
        {
            string line;
            while ((line = r.ReadLine()) != null)
            {
                listBox1.Items.Add(line);

            }
        }
    }