Lock

 private void button1_Click(object sender, EventArgs e)
        {
            Thread t = new Thread(new ThreadStart(this.testLock)); 
            t.Start();
        }
        private void button2_Click(object sender, EventArgs e)
        {
            Thread t = new Thread(new ThreadStart(this.testLock));
            t.Start();
        }       
        private void button3_Click(object sender, EventArgs e)
        {
            testLock();
        }
        private void testLock()
        {
            lock (this)
            {
                MessageBox.Show("code locked");
            }
        }