private void textBox2_Click(object sender, EventArgs e)
        {
            if (((TextBox)sender).Tag == null)
            {
                ((TextBox)sender).SelectAll();
            }
        }

------解决方案--------------------
那就这样

       private void textBox1_Enter(object sender, EventArgs e)
       {
           BeginInvoke((Action)delegate
           {
               textBox1.SelectAll();
           });
       }