1_A_CSHARP

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 _1_A_CSHARP
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.richTextBox1.Text = "Hello World!";
        }

        private void button2_Click(object sender, EventArgs e)
        {
            this.richTextBox1.Clear();
        }

        private void richTextBox1_MouseEnter(object sender, EventArgs e)
        {
            this.richTextBox1.BackColor = Color.Teal;
        }

        private void richTextBox1_MouseLeave(object sender, EventArgs e)
        {
            this.richTextBox1.BackColor = Color.Empty;
        }
    }
}