BeginInvoke and EndInvoke in thread

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace WindowsFormsApplication28
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {

            ParameterizedThreadStart start = new ParameterizedThreadStart(startthread);
            System.Threading.Thread u;
            int i = 0;
            while (i < 100)
            {
                u = new System.Threading.Thread(start);
                u.Start(i);

                Thread.Sleep(1000);


                lock (this)
                {
                    try
                    {
                        EndInvoke(g);   // return null??????why?
                    }
                    catch { }
                }

                i++;
            }
        }

        delegate void j(object h);
        j b;

        object h = new object();
        object y = new object();

        void startthread(object m)
        {
            lock (this)
            {
                b = new j(p1);

                if (label1.InvokeRequired)
                {
                    IAsyncResult g = label1.BeginInvoke(b, m);
                }
                else
                {
                    label1.Text = string.Format("Step is :{0}", h.ToString());
                }
            }
        }

        void p1(object h)
        {

            label1.Text = string.Format("Step is :{0}", h.ToString());

        }

        IAsyncResult g;
    }
}