Kompletní kód

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

namespace TextEdit01
{
public partial class oknoProgramu : Form
{
public oknoProgramu()
{
InitializeComponent();
}

private void btnOtevrit_Click(object sender, EventArgs e)
{
if (openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
{
label1.Text = openFileDialog1.FileName;
richTextBox1.Text = File.ReadAllText(label1.Text);
}
}

private void btnUlozit_Click(object sender, EventArgs e)
{
richTextBox1.SaveFile(label1.Text, RichTextBoxStreamType.PlainText);
}

private void btnNahradit_Click(object sender, EventArgs e)
{
int start = 0;
int konec = richTextBox1.Text.LastIndexOf(cbVyhledat.Text);

while (start < konec)
{
richTextBox1.Find(cbVyhledat.Text, start, richTextBox1.TextLength, RichTextBoxFinds.MatchCase);
richTextBox1.SelectionBackColor = Color.Yellow;
start = richTextBox1.Text.IndexOf(cbVyhledat.Text, start) + 1;

//Replace nefunguje
richTextBox1.Text = richTextBox1.Text.Replace(cbVyhledat.Text,
cbNahradit.Text);
}
}