c#异步操作的问题

//为什么textbox1上不显示内容，一定要等到执行完异步代码才在textbox1中显示,这段代码不是阻塞，怎么和阻塞的执行顺序一样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 WindowsApplication1
{
     public delegate DateTime AsyncExampleDelegate(int delay,string name);
    public partial class but_Blocking : Form
    {
        public but_Blocking()
        {
            InitializeComponent();
        }
        public static DateTime LongRunningMethod(int delay, string name)
        {
            but_Blocking myForm = new but_Blocking();
            myForm.textBox1.Text += DateTime.Now.ToString("HH:mm:ss:ffff") + name + "实例线程开始" + "\r\n";
            Console.WriteLine(myForm.textBox1.Text.ToString());
            Thread.Sleep(delay);
            myForm.textBox1.Text += DateTime.Now.ToString("HH:mm:ss:ffff") + name + "实例线程开始" + "\r\n";
            Console.WriteLine(myForm.textBox1.Text.ToString());
            //myForm.Show();
            return DateTime.Now;        }        private void but_Blocking_Click(object sender, EventArgs e)
        {
        }        private void button1_Click(object sender, EventArgs e)
        {
            if (this.textBox1.Text != "")
            {
                this.textBox1.Text = "";
            }
            this.textBox1.Text += Environment.NewLine + "***进行等待实例***" + "\r\n";            AsyncExampleDelegate longRunningMethod = new AsyncExampleDelegate(LongRunningMethod);
            IAsyncResult asyncResult = longRunningMethod.BeginInvoke(10000, "Waiting", null, null);
            this.textBox1.Text += DateTime.Now.ToString("HH:mm:ss:ffff") + "等待直到方法结束" + "\r\n";            while (!asyncResult.AsyncWaitHandle.WaitOne(200,false)) 
           {
                string aa=DateTime.Now.ToString("HH:mm:ss:ffff") + "等待超时" + "\r\n";
                this.textBox1.Text += aa;  //为什么textbox1上不显示内容，一定要等到执行完异步代码才在textbox1中显示
                Console.WriteLine(aa);
            }            DateTime completion = longRunningMethod.EndInvoke(asyncResult);
            this.textBox1.Text += completion.ToString("HH:mm:ss:ffff") + "等待实例结果" + "\r\n";
            this.textBox1.Text += completion.ToString();        }
    }
}