Step 1: Click New Project, then select Visual C# on the left, then Windows and then select Windows Forms Application. Name your project "SearchRichTextBox" and then click OK
Step 2: Design your form as below

Step 3: Add code to handle your form
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;
namespace SearchRichTextBox
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void btnSearch_Click(object sender, EventArgs e)
{
//Split a string to arrays
string[] words = txtSearch.Text.Split(',');
foreach(string word in words)
{
int startIndex = 0;
while (startIndex < richTextBox.TextLength)
{
//Find word & return index
int wordStartIndex = richTextBox.Find(word, startIndex, RichTextBoxFinds.None);
if (wordStartIndex != -1)
{
//Highlight text in a richtextbox
richTextBox.SelectionStart = wordStartIndex;
richTextBox.SelectionLength = word.Length;
richTextBox.SelectionBackColor = Color.Yellow;
}
else
break;
startIndex += wordStartIndex + word.Length;
}
}
}
private void btnClear_Click(object sender, EventArgs e)
{
//Clear
richTextBox.SelectionStart = 0;
richTextBox.SelectAll();
richTextBox.SelectionBackColor = Color.White;
}
}
}
VIDEO TUTORIALS