尝试一下：

在母版页中添加：

  <nav class="header-navbar navbar-expand-md navbar navbar-with-menu navbar-without-dd-arrow fixed-top navbar-semi-light">
        <div class="navbar-wrapper">
            <div class="navbar-container content">
                <div class="collapse navbar-collapse show" id="navbar-mobile">
                    <ul class="nav navbar-nav mr-auto float-left">
                        <li class="nav-item mobile-menu d-md-none mr-auto"><a class="nav-link nav-menu-main menu-toggle hidden-xs" href="#"><i class="ft-menu font-large-1"></i></a></li>
                        <li class="nav-item d-none d-md-block"><a class="nav-link nav-menu-main menu-toggle hidden-xs" href="#"><i class="ft-menu"></i></a></li>
                        <li class="nav-item d-none d-md-block"><a class="nav-link nav-link-expand" href="#"><i class="ficon ft-maximize"></i></a></li>
                    </ul>
                    <ul class="nav navbar-nav float-right">

                        <li class="nav-item"><span class="user-name text-bold-700 ml-1">
                            <asp:Label ID="UserName" runat="server" Text="" CssClass="form-control" Style="background: none !important; color: white !important; border: none!important"></asp:Label></span></li>
                        <li class="dropdown dropdown-user nav-item">

                            <a class="dropdown-toggle nav-link dropdown-user-link" href="#" data-toggle="dropdown">
                                <span class="avatar avatar-online">
                                    <img src="../../images/Admin.png" alt="Admin" />
                            </a>

                            <div class="dropdown-menu dropdown-menu-right">
                                <div class="arrow_box_right">

                                    <a class="dropdown-item" href="../../Logout.aspx"><i class="ft-power"></i>Logout</a>
                                </div>
                            </div>
                        </li>
                    </ul>
                </div>
            </div>
        </div>
    </nav>

C＃代码：

您应检查会话值是否存在，然后将显示名称；否则，如果会话为null，则重定向到登录页面

 protected void Page_Load(object sender,EventArgs e)
    {
      if (Session["User_ID"] == null)
       {
         Response.Redirect("~/Default.aspx");
       }
       else
       {
         UserName.Text = Session["Employee_Name"].ToString() + " (" + Session["User_ID"].ToString() + ")";
        }
     }