asp.net后台怎么读取到RadioButtonList单选框的值

                         像文本框string ID = this.txtID.Text.Trim();这样
                                                    <asp:RadioButtonList ID="RadioButtonList1"  runat="server" RepeatDirection="Horizontal" Width="400px" >
                                   <asp:ListItem Value="1">第一</asp:ListItem>

                                   <asp:ListItem Value="2">第二</asp:ListItem>
                              </asp:RadioButtonList>