使DataGrid中的CheckBox列全选(javacript)

javascript代码如下：

function SelectAll(spanChk)
        {
            // Added as ASPX uses SPAN for checkbox 
            var oItem = spanChk.children;
            var theBox=oItem.item(0)
            xState=theBox.checked;    

            elm=theBox.form.elements;
            for(i=0;i<elm.length;i++)
            if(elm[i].type=="checkbox" && elm[i].id!=theBox.id)
            {
                //elm[i].click();
                if(elm[i].checked!=xState)
                elm[i].click();
                //elm[i].checked=xState;
            }
        }

DataGrid中的调用方法如下：用红色标记        

<Columns>
                                <asp:TemplateColumn>
                                    <HeaderTemplate>
                                        <asp:CheckBox ID="AllChk" Runat="server" AutoPostBack="False" ToolTip="全选/全不选" onclick="javascript:SelectAll(this);"></asp:CheckBox>
                                    </HeaderTemplate>
                                    <ItemTemplate>
                                        <asp:CheckBox ID="Chk" Runat="server" AutoPostBack="False" onclick="HighlightRow(this)" OnCheckedChanged="Chk_CheckedChanged"></asp:CheckBox>
                                    </ItemTemplate>
                                 </asp:TemplateColumn>
                                <asp:BoundColumn HeaderText="主题" DataField="Topic"></asp:BoundColumn>
                                <asp:BoundColumn HeaderText="发布人" DataField="Person"></asp:BoundColumn>
                                <asp:BoundColumn HeaderText="发布时间" DataField="Time"></asp:BoundColumn>
                                <asp:BoundColumn HeaderText="状态" DataField="State"></asp:BoundColumn>
                            </Columns>