(C# Win App) สอบถามวิธีการเขียนคำสั่งเรียกฟอร์มลูกมาแสดงที่ฟอร์มแม่ครับ

        private void mnuCallFromA_Click(object sender, EventArgs e)
        {
            Form frm = new Form();
            Button b = new Button();

            b.Text = "Open Form Child";
            b.Location = new Point((this.Width - b.Width) / 2, (this.Height - b.Width) / 2);
            b.Click += new EventHandler(new Form1().mnuCallFromA_Click);

            frm.TopMost = true;
            frm.Controls.Add(b);
            frm.Show();
        }


        private void Form1_Load(object sender, EventArgs e)
        {
            Form frm = new Form();
            Button b = new Button();
            b.Text = "Open Form Child";
            b.Location = new Point((this.Width-b.Width) / 2, (this.Height-b.Width) / 2);
            b.Click += new EventHandler (new Form1().mnuCallFromA_Click);
            frm.TopMost = true;
            frm.Controls.Add(b);
            frm.Show();
            this.WindowState = FormWindowState.Maximized;
            this.IsMdiContainer = true;

        }

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 WindowsFormsApplication1
{
    public partial class FormMain : Form
    {
        public FormMain()
        {
            InitializeComponent();
        }

        
        private void FormMain_Load(object sender, EventArgs e)
        {
            this.WindowState = FormWindowState.Maximized;
            this.IsMdiContainer = true;
            FormA frmFormA = new FormA();
            frmFormA.frmMain = this;
            frmFormA.Show();

        }
    }

    public class FormA : Form
    {
        private Button b;
        private FormA frmFormB;
        public FormMain frmMain;
        public FormA()
        {
            this.TopMost = true;
            this.MdiParent =  frmMain;
            b = new Button();
            b.Text = "Open Form";
            b.Location = new Point((this.Width - b.Width) / 2, (this.Height - b.Height) / 2);
            b.Click += new EventHandler(b_Click);
            this.Controls.Add(b);
        }

        void b_Click(object sender, EventArgs e)
        {
            frmFormB = new FormA();
            frmFormB.Show();
            this.Close();
        }


    }

}