The name 'MessageBox' does not exist in the current context

public class Class1
   {


       public static void OpenConnection()
       {
           SqlConnection cn;
           cn = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename='D:\My_Project\My_Project\DATABASE\Soneri_Bank_loan_system.mdf';Integrated Security=True;Connect Timeout=30;User Instance=True");
               try
               {
                   cn.Open();
               }
               catch (Exception e1)
               {

                   MessageBox.Show(e1.Message);
               }
           }

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
    public Form1()
    {
        InitializeComponent();
    }

    private void Form1_Load(object sender, System.EventArgs e)
    {
        Class1.OpenConnection();//this is your Static function that u have created in Class 1//
    }

   
    }
}

public class Class1
   {

public static SqlConnection cn;
	cn = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename='D:\My_Project\My_Project\DATABASE\Soneri_Bank_loan_system.mdf';Integrated Security=True;Connect Timeout=30;User Instance=True");

       public static void OpenConnection()
       {
           
               try
               {
                   cn.Open();
               }
               catch (Exception e1)
               {

                   MessageBox.Show(e1.Message);
               }
           }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace WindowsFormsApplication1.code
{
    class Class1
    {

        public static SqlConnection con = new SqlConnection("Data Source=.\\sqlexpress;Integrated Security=True");
        public Class1()
        {

            //
            // TODO: Add constructor logic here
            //
        }
        public static void Open()
        {
            try
            {
                con.Open();
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }


    }
}

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 WindowsFormsApplication1.code;
using System.Data.SqlClient;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Class1.Open();
            string query = "Select * from Table1";
            SqlDataAdapter da = new SqlDataAdapter(query, Class1.con);
            DataTable dt = new DataTable(); da.Fill(dt); 
            if (dt.Rows.Count > 0) 
            { 
                MessageBox.Show("jhdfj", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error); 
               
            }
        }
    }
}