มีปัญหาในการใช้ MDI โดยภาษา C# ปัญหาการส่งค่าจากฟอร์มลูกไปฟอร์มหลักครับ

//FormMain
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 Project_1
{
public partial class FormMain : Form
{
public FormMain()
{
InitializeComponent();
FormLogin f = new FormLogin(this);
f.Show();
}

private void FormMain_Load(object sender, EventArgs e)
{
toolStrip1.Enabled = false;
}
}
}

//FormLogin
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;
using System.Data.SqlClient;
using System.IO;

namespace Project_1
{
public partial class FormLogin : Form
{
public FormLogin(FormMain containerForm)
{
InitializeComponent();
MdiParent = containerForm;
}
SqlConnection Conn = new SqlConnection();

private void FormLogin_Load(object sender, EventArgs e)
{
DBConnString Connstring = new DBConnString();
string strConn = Connstring.strConn;

if (Conn.State == ConnectionState.Open)
{
Conn.Close();
}

Conn.ConnectionString = strConn;
Conn.Open();
}

private void btnOk_Click(object sender, EventArgs e)
{
StringBuilder sb = new StringBuilder();
sb.Append("SELECT * FROM Employee");
sb.Append(" WHERE (EmpUsername='" + txtUsername.Text.Trim() + "')");
sb.Append(" AND (EmpPassword='" + txtPassword.Text.Trim() + "')");

string sqlLogin;
sqlLogin = sb.ToString();

SqlCommand com = new SqlCommand();
SqlDataReader dr;

com.CommandType = CommandType.Text;
com.CommandText = sqlLogin;
com.Connection = Conn;

dr = com.ExecuteReader();

if (dr.HasRows)
{
this.Hide();
}
else
{
MessageBox.Show("Username หรือ Password ที่คูณป้อนไม่ถูกต้อง !!!", "ผลการทำงาน", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
txtUsername.Text = "";
txtUsername.Focus();
txtPassword.Text = "";
}

dr.Close();
dr.Dispose();
}

private void btnCancel_Click(object sender, EventArgs e)
{
Application.Exit();
}
}
}