6. 搜尋字串並標示黃色背景顏色

namespace Q6
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            openFileDialog1.Filter = "txt files (*.txt)|*.txt";
            openFileDialog1.ShowDialog();
            richTextBox1.Text = new StreamReader(openFileDialog1.OpenFile()).ReadToEnd();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var to = textBox1.Text;
            if (to.Length == 0)
            {
                MessageBox.Show("未輸入欲搜尋的字串");
                return;
            }
            int i = 0;
            int n = 0;
            richTextBox1.SelectAll();
            richTextBox1.SelectionBackColor = Color.White;

            while ((i = richTextBox1.Text.IndexOf(to, i)) != -1)
            {
                richTextBox1.Select(i, to.Length);
                richTextBox1.SelectionBackColor = Color.Yellow;
                i = i + to.Length;
                n++;
            }
            amt.Text = n.ToString();
        }
    }
}