Menu

C#

protected void rptMenuParent_OnItemDataBound(object sender, RepeaterItemEventArgs e)
{
    var row = (TopMenu)e.Item.DataItem;

    Repeater nestedRepeater = e.Item.FindControl("rptMenuChild") as Repeater;
    nestedRepeater.DataSource = row.Children;
    nestedRepeater.DataBind();
}

protected void Page_Load(object sender, EventArgs e)
{
    var menuItems = new List<TopMenuItem>();
    
    // add menu items
    
    rptMenuParent.DataSource = menuItems;
    rptMenuParent.DataBind();
}

Menu Object

public class TopMenu
{
    public MenuItem Parent { get; set; }

    public List<MenuItem> Children { get; set; }
}

public class MenuItem
{
    public string Link { get; set; }

    public string Text { get; set; }
}

Nested repeater

<ul id="menu-header" class="menu">
    <asp:Repeater ID="rptMenuParent" runat="server" OnItemDataBound="rptMenuParent_OnItemDataBound">
        <itemtemplate>
            <li class="menu-item">
                <a href="<%# Eval("Parent.Link") %>"><%# Eval("Parent.Text") %></a>
                <asp:Repeater ID="rptMenuChild" runat="server">
                    <HeaderTemplate><ul class="sub-menu"></HeaderTemplate> 
                    <itemtemplate>
                        <li class="menu-item">
                            <a href="<%# Eval("Link") %>"><%# Eval("Text") %></a>
                        </li>
                    </itemtemplate>
                    <FooterTemplate></ul></FooterTemplate>
                </asp:Repeater>
            </li>
        </itemtemplate>
    </asp:Repeater>
</ul>