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.Threading;
using System.Windows.Forms;

namespace WindowsFormsApp1
{
	public partial class Form1 : Form
	{
		public Form1()
		{
			InitializeComponent();
		}

		private void button1_Click(object sender, EventArgs e)
		{
			Thread func = new Thread(lib.test_func);
			func.Start(this);
		}
	}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WindowsFormsApp1
{
	public static class lib
	{
		public static void test_func(object frm)
		{
			var frm2 = frm as Form1;
			while (true)
			{
				System.Threading.Thread.Sleep(100);
				frm2.Invoke(new Action(() => {
					frm2.Text = DateTime.Now.ToString();
				}));

			}
		}

	}
}