textBox to DateTime in DB

Iam trying to get a DateTime out of an textBox, where it allready is in Format of MySql DateTime. The column in the DB is also DateTime format.

However, when i press my button to save the Dates in the DB, the whole row is gonna get emptyed.

I tried around with different formats und DataTypes in DB without anny effect

private void button4_Click(object sender, EventArgs e)
{
    MySqlConnection conn = DBUtils.GetDBConnection();
    conn.Open();

    string startzeit = textBoxstartzeit.Text.ToString();
    DateTime start = DateTime.Parse(startzeit); 
    string stopzeit = textBoxstopzeit.Text.ToString();
    DateTime stop = DateTime.Parse(stopzeit);
    string pstartzeit = textBoxstopzeit.Text.ToString();
    DateTime pstart = DateTime.Parse(pstartzeit);
    string pstopzeit = textBoxstopzeit.Text.ToString();
    DateTime pstop = DateTime.Parse(pstopzeit);

    MySqlCommand cmdnew = conn.CreateCommand();
    cmdnew.CommandType = CommandType.Text;
    cmdnew.CommandText = "UPDATE arbeitszeiten SET astart = '" + start + "',  astop = '" + stop + "', pstart = '" + pstart + "', pstop = '" + pstop + "' WHERE id = '" + dataGridView.CurrentCell.Value + "'";
    cmdnew.ExecuteNonQuery();

    conn.Close();
}

private void dataGridView_CellClick(object sender, DataGridViewCellEventArgs e)
{
    try
    {
        MySqlConnection conn = DBUtils.GetDBConnection();
        conn.Open();

        MySqlCommand feedstartzeit = conn.CreateCommand();
        feedstartzeit.CommandText = "SELECT astart FROM arbeitszeiten WHERE id = '" + dataGridView.CurrentCell.Value + "'";
        DateTime start = Convert.ToDateTime(feedstartzeit.ExecuteScalar());
        textBoxstartzeit.Text = start.ToString("yyyy-MM-dd HH:mm:ss");

        MySqlCommand feedstopzeit = conn.CreateCommand();
        feedstopzeit.CommandText = "SELECT astop FROM arbeitszeiten WHERE id = '" + dataGridView.CurrentCell.Value + "'";
        DateTime stop = Convert.ToDateTime(feedstopzeit.ExecuteScalar());
        textBoxstopzeit.Text = stop.ToString("yyyy-MM-dd HH:mm:ss");

        MySqlCommand feedstartpause = conn.CreateCommand();
        feedstartpause.CommandText = "SELECT pstart FROM arbeitszeiten WHERE id = '" + dataGridView.CurrentCell.Value + "'";
        DateTime startpause = Convert.ToDateTime(feedstartpause.ExecuteScalar());
        textBoxstartpause.Text = startpause.ToString("yyyy-MM-dd HH:mm:ss");

        MySqlCommand feedstoppause = conn.CreateCommand();
        feedstoppause.CommandText = "SELECT pstop FROM arbeitszeiten WHERE id = '" + dataGridView.CurrentCell.Value + "'";
        DateTime stoppause = Convert.ToDateTime(feedstoppause.ExecuteScalar());
        textBoxstoppause.Text = stoppause.ToString("yyyy-MM-dd HH:mm:ss");
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message, "Bitte ID auswählen", MessageBoxButtons.OK, MessageBoxIcon.Error);
    }
}

Button4 is the upload new data and the dataGridView part is filling the textBoxes with a preformated datetime that later get uploaded by button4

Answer

Allright, the by Jon Skeet suggested parametered sql request solved the problem.