C# clicker durdurma butonu

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Windows.Input;

namespace end
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        [DllImport("user32.dll")]
        static extern void mouse_event(int dwFlags, int dx, int dy, int dwdata, int dwextrainfo);
        public enum mouseeventFlags
        {
            LeftDown = 2,
            LeftUp = 4,
        }
        public void leftclick(Point p)
        {
            mouse_event((int)(mouseeventFlags.LeftDown), p.X, p.Y, 0, 0);
            mouse_event((int)(mouseeventFlags.LeftUp), p.X, p.Y, 0, 0);
        }
        bool stop = true;
        private void button1_Click(object sender, EventArgs e)
        {
            stop = (stop) ? false : true;
            timer1.Interval = (int)numericUpDown1.Value;
            timer1.Enabled = true;
            if (!stop) timer1.Start();
            if (stop) timer1.Stop();
        }
        private void timer1_Tick(object sender, EventArgs e)
        {
            leftclick(new Point(MousePosition.X, MousePosition.Y));
        }


      
     

        private void Form1_Load(object sender, EventArgs e)
        {
               
        }
    }
}