Saving ListBox items in C#

using System.IO;
using System.Diagnostics;
namespace WindowsFormsApplication1
{
    public partial class MainForm : Form
    {
    public MainForm()
    {

        InitializeComponent();
    }
private void btnSubmit_Click(object sender, EventArgs e)
{
    listBox.Items.Add(txtInput.Text)
    txtInput.Clear();
}
private void btnSaveToFile_Click(object sender, EventArgs e)
{
    if (listBox.Items.Count > 0)
{
    {
    using (TextWriter TW = new StreamWriter("list.txt"))
    {
        foreach (string itemText in listBox.items)
    {
            TW.WriteLine(itemText);
    }
}
    Process.Start("list.txt")
}
private void btnCopyToClip_Click(object sender. EventArgs e)
{
    if (listBox.Items.Count > 0)
    {
        StringBuilder SB = new StringBuilder();
        foreach (string itemValue in listBox.Items)
        {
        SB.AppendLine(itemValue);
        }

    //Further process text here, ex remove last newline https://www.youtube.com/watch?v=aIeiVfzJ1sU
    string result = SB.ToString().TrimEnd('\n');

    Clipboard.SetText(result);
    }
}
}
}
// textinputbox, submit, listbox, copytoclipboard, savetofile.