5. 設計一字串搜尋程式

namespace Q4
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox4_TextChanged(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            var txt = File.ReadAllText(textBox1.Text);
            textBox4.Text = txt;

            var idx = txt.Replace("\r\n", "").IndexOf(textBox2.Text);
            textBox3.Text = idx == -1 ? "未找到 " + textBox2.Text + " 字串" : (idx + 1).ToString();
        }
    }
}