关于一个子窗体向父窗体传值的问题


1 我的父窗体 是点击按钮打开一个窗体。

  <asp:Button ID="btnChoseUnit" OnClientClick="javascript:OpenPopup();" runat="server"

                        Text="单位选择" />

<input type="hidden" id="txtUnitName" runat="server" />

                    <input type="hidden" id="txtUnitId" runat="server" />

js代码

<script type="text/javascript">

        function OpenPopup() {



            window.showModalDialog("TreeViewInfo.aspx", "List", "scrollbars=no,resizable=no,width=400,height=263");

        }

        function setValue(UnitName, UnitId) {

            document.getElementById('txtUnitName').value = UnitName;

            document.getElementById('txtUnitId').value = UnitId;

        }

    </script>

看了网上的好多参考，却不知道 
<input type="hidden" id="txtUnitName" runat="server" />
                    <input type="hidden" id="txtUnitId" runat="server" />

这两个隐藏控件的用处。

2 我的子窗体是一个目录树

<input type="hidden" runat="server" id="txtUnitName" />

    <input type="hidden" runat="server" id="txtUnitId" />

    <div>

        <asp:TreeView ID="TrView" Font-Bold="true" Font-Size="15px" runat="server" ImageSet="Contacts"

            ShowCheckBoxes="All" OnTreeNodeCheckChanged="TrView_TreeNodeCheckChanged">

            <ParentNodeStyle Font-Bold="False" />

            <HoverNodeStyle Font-Underline="True" ForeColor="#5555DD" />

            <SelectedNodeStyle Font-Underline="True" HorizontalPadding="0px" VerticalPadding="0px"

                ForeColor="#5555DD" />

            <NodeStyle Font-Names="Verdana" Font-Size="8pt" ForeColor="Black" HorizontalPadding="5px"

                NodeSpacing="0px" VerticalPadding="0px" />

        </asp:TreeView>

    </div>

    <div>

        <asp:Button ID="btnUnitAdd" runat="server" Text="确认选择" OnClick="btnUnitAdd_Click" />

    </div>

&nb