程序代码：

        int Search(string s, string p, int idx)
        {
            if (s.Length < p.Length) return -1;

            for (int i = idx; i <= s.Length - p.Length; i++)
            {
                bool isOk = true;
                for (int j = 0; j < p.Length; j++)
                {
                    if (s[i + j] != p[j])
                    {
                        isOk = false;
                        break;
                    }
                }
                if (isOk) return i;
            }

            return -1;
        }

        int Search(string s, string p)
        {
            return Search(s, p, 0);
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            string s = "123456789123456789";
            string p = "456";

            this.Text = Search(s, p) + " " + Search(s, p, 4) + " " + s.IndexOf(p) + " " + s.IndexOf(p, 4);
            //结果[3 12 3 12]
        }