cross thread - c#

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.Timers;

namespace snake
{

    public partial class Form1 : Form
    {
        public static int dir = 1, x, y;
        public static Point p;
        public Form1()
        {
            InitializeComponent();
            p = blacksq.Location;
        }
        
        private void Form1_Load(object sender, EventArgs e)
        {
            Timer();
        }
        
        public void Timer()
        {
            System.Timers.Timer aTimer = new System.Timers.Timer();
            aTimer.Elapsed += new ElapsedEventHandler(onTimedEvent);
            // time in ms = 4 moves per sec
            aTimer.Interval = 250;
            aTimer.Enabled = true;

            MessageBox.Show("timer started");
            
        }

        //onTimedEvent - move snake
        public void onTimedEvent(object source, ElapsedEventArgs e)
        {
            Point p = blacksq.Location;
            // x = east/west
            // y = north/south
            if (dir == 0)
            {
                //move north
                p.Y += 5;
                //blacksq.Location = p;
                Location = p;

            }
            else if (dir == 1)
            {
                //move east
                p.X += 5;
                Location = p;
            }
            else if (dir == 2)
            {
                //move south
                p.Y -= 5;
                Location = p;
            }
            else if (dir == 3)
            {
                //move west
                p.X -= 5;
                Location = p;
            };
        }

        private void Form1_KeyDown(object sender, KeyEventArgs e)
        {
            // on keypress (arrows) change direction
            if (e.KeyCode == Keys.Right)
            {
                dir = 0;
            }
            else if (e.KeyCode == Keys.Up)
            {
                dir = 1;
            }
            else if (e.KeyCode == Keys.Left)
            {
                dir = 2;
            }
            else if (e.KeyCode == Keys.Down)
            {
                dir = 3;
            }
        }
    }
}