在Form1中  textbox1的modifilers設為Public,
      public static Form1 f=null;
      public Form1()
        {
            InitializeComponent();
            f = this;
       }
      private void button1_Click(object sender, EventArgs e)
      {
          test cl = new test();
          cl.t_test();         
      }  

  類檔案:
  public class test
    {           
        public void t_test()
        {
            Form1.f.textBox1.Text = "this is a test message!"; 
        }
       
    }