Opening a file in Notepad

Hello. There was a problem when opening a program file in notepad.

        string tmp = listBox1.SelectedItem.ToString();
        tmp = tmp.Replace("\\", "\\\\");
        Process.Start("C:\\Windows\\System32\\notepad.exe", tmp);

When you double-click on a line in the listbox, notepad opens and the error "Syntax error in the file name, folder name, or volume label." This is very strange, because I escape slashes and there should be no error...

I tried to create a string, and write the path to the file in it (naturally escaping slashes, a string like " C:\file.txt"), the file is wonderful I opened it in my notebook and this fact completely baffled me... What is the error and what I am doing wrong, I do not understand...

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Diagnostics;

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

        private void button1_Click(object sender, EventArgs e)
        {
            progressBar1.Step = 0;
            progressBar1.PerformStep();

            string TmpFile = "";
            int NumberFiles = 0;
            List<string> FileOfName = new List<string>();

            System.String path = PathFile.Text;

            try
            {
                foreach (var file in Directory.GetFiles(path, SelectType.SelectedItem.ToString(), SearchOption.AllDirectories))
                {
                    StreamReader FindFile = new StreamReader(file);
                    string str = "";
                    while (!FindFile.EndOfStream)
                    {
                        str = FindFile.ReadLine();
                        if ((str.IndexOf(Substring.Text) > -1) && !TmpFile.Equals(file))
                        {
                            FileOfName.Add(file);
                            TmpFile = file;
                            NumberFiles++;
                        }
                    }    
                }

                if (NumberFiles != 0)
                {
                    int StepProgressBar = NumberFiles / 100;
                    progressBar1.Step = StepProgressBar;

                    foreach (string s in FileOfName)
                    {
                        listBox1.Items.Add(s + "\n");
                        progressBar1.PerformStep();
                    }                        
                }
                else
                {
                    progressBar1.Step = 100;
                    progressBar1.PerformStep();
                    MessageBox.Show("По указанному пути введенная вами строка не найдена");
                }
            }
            catch (Exception error)
            {
                MessageBox.Show(error.ToString());
            }
        }

        private void listBox1_DoubleClick(object sender, EventArgs e)
        {
            string tmp = listBox1.SelectedItem.ToString();
            //tmp = tmp.Replace("\\", "\\\\");
            MessageBox.Show(tmp);
            Process.Start("C:\\Windows\\System32\\notepad.exe", tmp);
        }
    }
}