Start and Stop Thread using button

Question

I'm trying to start thread and then end thread by clicking buttons in Winform but the MessageBox does not appear after clicking the Stop button( which is the button 3 here). \

Here's my code:

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.Threading;

namespace ThreadForm
{
    public partial class Form1 : Form
    {
        public static volatile bool isPausePressed = false;
        Thread SequenceTest = new Thread(DoWork);
        Thread Seq1Test = new Thread(TestSequence1);
        Thread Seq2Test = new Thread(TestSequence2);
        Thread Seq3Test = new Thread(TestSequence3);
        

        public Form1()
        {
            InitializeComponent();
        }
        Form2 objform;

        private void button1_Click(object sender, EventArgs e)
        {
            objform = new Form2();
            objform.Show();
            var formWithButton = new Form2();
            formWithButton.Btn_Change.BackColor = Color.White;
            SequenceTest.Start();
            Seq1Test.Start();
            Seq2Test.Start();
            Seq3Test.Start();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            isPausePressed = true;
        }

        private void button3_Click(object sender, EventArgs e)
        {
            bool a, b, c, d;
            a = b = c = d = false;
            SequenceTest.Join();
            /*if (SequenceTest.ThreadState == System.Threading.ThreadState.Stopped)
                a = true;*/
            Seq1Test.Join();
            /*if (Seq1Test.ThreadState == System.Threading.ThreadState.Stopped)
                b = true;*/
            Seq2Test.Join();
            /*if (Seq2Test.ThreadState == System.Threading.ThreadState.Stopped)
                c = true;*/
            Seq3Test.Join();
            /*if (Seq3Test.ThreadState == System.Threading.ThreadState.Stopped)
                d = true;*/
            MessageBox.Show("Done!", "ThreadTest");
        }

        public static void DoWork()
        {
            Thread.Sleep(1000);
            //Console.WriteLine("Current main task working on thread: {0}", Thread.CurrentThread.ManagedThreadId);
            while (!isPausePressed)
            {
                Thread.Sleep(100);
            }
        }

        public static void TestSequence1()
        {
            int i = 0;
            Thread.Sleep(1000);
            //Console.WriteLine("Sequence1 working on thread: {0}", Thread.CurrentThread.ManagedThreadId);
            while (true)
            {
                i++;
                //Console.WriteLine("Current i value: {0}", i);
            }
        }

        public static void TestSequence2()
        {
            int j = 0;
            //Console.WriteLine("Sequence2 working on thread: {0}", Thread.CurrentThread.ManagedThreadId);
            Thread.Sleep(1000);
            while (true)
            {
                j++;
                //Console.WriteLine("Current j value: {0}", j);
            }
        }

        public static void TestSequence3()
        {
            int k = 0;
            Thread.Sleep(1000);
            //Console.WriteLine("Sequence3 working on thread: {0}", Thread.CurrentThread.ManagedThreadId);
            while (true)
            {
                k++;
                //Console.WriteLine("Current k value: {0}", k);
            }
        }
    }
}