<asp:HyperLink ID="HyperLink1" runat="server" NavigateUrl="InBox.aspx" Text="Входящие"></asp:HyperLink>
                    <asp:HyperLink ID="HyperLink2" runat="server" NavigateUrl="OutBox.aspx" Text="Исходящие"></asp:HyperLink>
                    <asp:HyperLink ID="HyperLink3" runat="server" NavigateUrl="~WebUsers/Archive.aspx" Text="Архив"></asp:HyperLink>

 private void SelectMenuItem(Control parentControl)
    {
        foreach (var link in parentControl.Controls.OfType<HyperLink>())
        {
            if (Request.Url.AbsolutePath.Contains(string.IsNullOrEmpty(link.NavigateUrl) ? "Staff.aspx" : link.NavigateUrl))
            {
                link.CssClass = "selected";
                break;
            }
        }
    }