Замена строки без изменения кодировки - C#

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;
 
namespace AutoReplaceReport
{
    public partial class frm1 : Form
    {
        public frm1()
        {
            InitializeComponent();
        }
 
        private void btnEnter_Click(object sender, EventArgs e)
        {
            if (File.Exists("Report_j13.txt"))
            {
                string str = string.Empty;
                using (StreamReader reader = File.OpenText(@"Report_j13.txt"))
                {
                    str = reader.ReadToEnd();
                }
                str = str.Replace("{rab_rob}", txtRab.Text);
 
                using (StreamWriter file = new StreamWriter(@"Report_j13.txt"))
                {
                    file.Write(str);
                }
            }
            else
            {
                MessageBox.Show("Файл не найден");
            }
        }
    }
}