namespace WindowsFormsApplication1
{
    public partial class Form1 : System.Windows.Forms.Form
    {
        public Form1()
        {
            InitializeComponent();
 
            // create list of data
            var _List = new System.Collections.Generic.List<Data> 

private void dataGridViewOrders_SelectionChanged(object sender, EventArgs e)
{
    dataGridViewOrderDetails.DataSource = null;
    if (dataGridViewOrders.SelectedRows.Count == 0) return;
 
    DataGridViewRow row = dataGridViewOrders.SelectedRows[0];
    if (row == null) return;
 
    int orderId = int.Parse(row.Cells["OrderId"].Value.ToString());