using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace VirtualMode
{
  partial class VirtualModeForm : Form
  {
   private List<DataObject> m_Data = new List<DataObject>();
   private List<bool> m_Visited = new List<bool>();
   public VirtualModeForm()
   {
     InitializeComponent();
     m_Grid.CellValueNeeded += OnCellValueNeeded;
     m_GetVisitedCountButton.Click += OnGetVisitedCount;
     InitData();
     InitGrid();
   }
   private void InitData()
   {
     for (int i = 0; i < 1000001; i++)
     {
      m_Visited.Add(false);
      DataObject obj = new DataObject();
      obj.Id = i;
      obj.Val = 2 * i;
      m_Data.Add(obj);
     }
   }
   private void InitGrid()
   {
     m_Grid.VirtualMode = true;
     m_Grid.ReadOnly = true;
     m_Grid.AllowUserToAddRows = false;
     m_Grid.AllowUserToDeleteRows = false;
     m_Grid.ColumnCount = 3;
     m_Grid.Rows.Add();
     m_Grid.Rows.AddCopies(0, 1000000);
     // Uncomment the next line and comment out the 
     // the rest of the method to switch to data bound mode
     //m_Grid.DataSource = m_Data;
   }
   private void OnCellValueNeeded(object sender,
     DataGridViewCellValueEventArgs e)
   {
     m_Visited[e.RowIndex] = true;
     if (e.ColumnIndex == 0)
     {
      e.Value = m_Data[e.RowIndex].Id;
     }
     else if (e.ColumnIndex == 1)
     {
      e.Value = m_Data[e.RowIndex].Val;
     }
     else if (e.ColumnIndex == 2)
     {
      Random rand = new Random();
      e.Value = rand.Next();
     }
   }
   private void OnGetVisitedCount(object sender, EventArgs e)
   {
     int count = 0;
     foreach (bool b in m_Visited)
     {
      if (b) count++;
     }
     MessageBox.Show(count.ToString());
   }
    private void VirtualModeForm_Load(object sender, EventArgs e)
    {
    }
  }
  public class DataObject
  {
   private int m_Id;
   private int m_Val;
   public int Val
   {
     get { return m_Val; }
     set { m_Val = value; }
   }
   public int Id
   {
     get { return m_Id; }
     set { m_Id = value; }
   }
  }
}