Form1上,点击某个按钮,弹出 Form2, 
               Form2 frm2=new Form2();
               DialogResult dr=frm2..ShowDialog();
                if (dr == DialogResult.OK)
                {
                     // Form2中,有个ComboBox控件,如何获取Form2 中ComboBox 选择的项,内容?
                }
                else if (dr == DialogResult.Cancel)
                {
                    continue;
                }