private void button1_Click(object sender, EventArgs e)
{
    if (richTextBox1.Find("as",
        richTextBox1.SelectionStart + richTextBox1.SelectionLength,
        richTextBox1.TextLength, RichTextBoxFinds.None) < 0) // 为-1表示没有找到
    {
        richTextBox1.SelectionStart = 0; // 光标设置为0位置
        richTextBox1.SelectionLength = 0;
    }
    richTextBox1.Focus();
}

        private void button1_Click(object sender, EventArgs e)
        {
            string find = this.textBox1.Text;
            int index = this.richTextBox1.Find(find);
            if (index < 0)
            {
                MessageBox.Show("查找不到要搜索的字符串");
                return;
            }
            this.richTextBox1.SelectionStart = index;
            this.richTextBox1.SelectionLength = find.Length;
            this.richTextBox1.Focus();
        }