Application.DoEvents

  private void button1_Click(object sender, EventArgs e)
        {
           Application.DoEvents();
            this.label1.Text = "步骤1";
            Application.DoEvents();
            System.Threading.Thread.Sleep(5000);

           Application.DoEvents();
            this.label1.Text = "步骤2";
            Application.DoEvents();
            System.Threading.Thread.Sleep(5000);

           Application.DoEvents();
            this.label1.Text = "步骤3";
            Application.DoEvents();
            System.Threading.Thread.Sleep(5000);

            Application.DoEvents();
            this.label1.Text = "完成";
            Application.DoEvents();
          
        }