public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        SqlConnection con = new SqlConnection("connstr");
        private void button1_Click(object sender, EventArgs e)
        {
            int index = dataGridView1.SelectedRows[0].Index;
            if (dataGridView1.SelectedRows.Count > 0)
            {

                using (SqlCommand cmd = con.CreateCommand())
                {
                  
                    int id = Convert.ToInt32(dataGridView1.SelectedRows[0].Cells[0].Value);
                    cmd.CommandText = "Delete from [Student] where id='" + id + "'";


                    con.Open();
                    cmd.ExecuteNonQuery();
                    con.Close();

                    for (int i = index + 1; i < dataGridView1.Rows.Count; i++)
                    {
                        UpdateId(i);

                    }
                    DisplayData();

                    dataGridView1.Refresh();
                    //commentbox.Text = "削除しました ";
                    //commentbox.BackColor = Color.Yellow;

                }
            }
            else
            {
                //commentbox.Text = "削除する生を選択してください ";
                //commentbox.BackColor = Color.Yellow;

            }
        }

        private void UpdateId(int id)
        {
            con.Open();
            string str = string.Format("Update Student set id={0} where id={1} ", id - 1, id);
            SqlCommand cmd = new SqlCommand();
            cmd.CommandText = str;
            cmd.Connection = con;
            cmd.ExecuteNonQuery();
            con.Close();

        }
        //display

        private void DisplayData()

        {
            con.Open();
            DataTable dt = new DataTable();
            SqlDataAdapter adp = new SqlDataAdapter("SELECT * FROM [Student] ", con);
            adp.Fill(dt);
            dataGridView1.DataSource = dt;


            con.Close();


        }
      

        private void Form1_Load(object sender, EventArgs e)
        {
            DisplayData();
        }
    }