Highlights Text in a Text File in C#

using System.IO;

        private void button1_Click(object sender, EventArgs e)
        {
 
            OpenFileDialog ofd = new OpenFileDialog();
 
            if (ofd.ShowDialog() == DialogResult.OK) 
            {
                //get the filename of the text file
                string filename = ofd.FileName;
                //read all the contents
                string textfile = File.ReadAllText(filename);
                // display the contents in the richtextbox
                rtDisplay.Text = textfile;
            }
 
        }

        private void button2_Click(object sender, EventArgs e)
        {
            //string words = txtSearch.Text;
            //rtDisplay.Text = "";
            //rtDisplay.Text = words;
            int index = 0;
 
            // if the textbox has an inputted text 
            while (index < rtDisplay.Text.LastIndexOf(txtSearch.Text)){
                //find text in the searched word
                rtDisplay.Find(txtSearch.Text,rtDisplay.TextLength, RichTextBoxFinds.None);
                // highlights the word with a color yellow
                rtDisplay.SelectionBackColor = Color.Yellow;
 
                // highlights all those words that is the same with the searched word in the textbox
                index = rtDisplay.Text.IndexOf(txtSearch.Text, index) + 1;
            }
 
        }