Form2 form2 = new Form2(this);
form2.Show();

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace daniweb.checkboxes
{
  public partial class frmMain : Form
  {
    private frmOther frm;
 
    public frmMain()
    {
      InitializeComponent();
    }
 
    private void frmMain_Load(object sender, EventArgs e)
    {
      frm = new frmOther();
    }
 
    private void buttonShowOtherForm_Click(object sender, EventArgs e)
    {
      frm.Show();
    }
 
    private void buttonHideOtherForm_Click(object sender, EventArgs e)
    {
      frm.Hide();
    }
 
    private void buttonCheckAll_Click(object sender, EventArgs e)
    {
      frm.SetCheckValue(true);
    }
 
    private void buttonUncheckAll_Click(object sender, EventArgs e)
    {
      frm.SetCheckValue(false);
    }
  }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
namespace daniweb.checkboxes
{
  public partial class frmOther : Form
  {
    public frmOther()
    {
      InitializeComponent();
    }
 
    private void frmOther_FormClosing(object sender, FormClosingEventArgs e)
    {
      e.Cancel = true;
      this.Hide();
    }
 
    public void SetCheckValue(bool Checked)
    {
      foreach (Control c in this.Controls)
      {
        SetCheckValue(Checked, c);
      }
    }
 
    private static void SetCheckValue(bool Checked, Control c)
    {
      CheckBox cb = (c as CheckBox);
      if (cb != null)
        cb.Checked = Checked;
      foreach (Control childCtrl in c.Controls)
      {
        SetCheckValue(Checked, childCtrl);
      }
    }
 
  }
}