C# อยากทราบเรื่อง การส่งค่าข้าม Form ที่เป็น MDIparent and child

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 WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.IsMdiContainer = true;
        }

        private void addToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Form2 frm = new Form2();
            frm.MdiParent = this;
            frm.Show();
        }

        private void closeToolStripMenuItem1_Click(object sender, EventArgs e)
        {
            this.ActiveMdiChild.Close();
        }

        private void saveToolStripMenuItem_Click(object sender, EventArgs e)
        {
            ((Form2)this.ActiveMdiChild).Save();
        }
    }
}

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 WindowsFormsApplication2
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        public void Save()
        {
            MessageBox.Show("Save file.");
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form3 frm = new Form3();
            frm.ShowDialog();
            textBox1.Text = frm.EmployeeID;
        }
    }
}

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 WindowsFormsApplication2
{
    public partial class Form3 : Form
    {
        public string EmployeeID { get; set; }

        public Form3()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            EmployeeID = textBox1.Text;
            this.Close();
        }
    }
}