多级导航栏选项

Question

我有一个“ ASP.Net Core2.1 MVC”（带有Bootstrap 4）项目。

我需要菜单栏中的第三级导航。 我遵循了一个codepen example，但是它不起作用。 （我认为该示例用于Bootstrap 3吗？）

CSS代码示例为：

.dropdown-submenu {
    position: relative;
}

.dropdown-submenu>.dropdown-menu {
    top: 0;
    left: 100%;
    margin-top: -6px;
    margin-left: -1px;
    -webkit-border-radius: 0 6px 6px 6px;
    -moz-border-radius: 0 6px 6px;
    border-radius: 0 6px 6px 6px;
}

.dropdown-submenu:hover>.dropdown-menu {
    display: block;
}

.dropdown-submenu>a:after {
    display: block;
    content: " ";
    float: right;
    width: 0;
    height: 0;
    border-color: transparent;
    border-style: solid;
    border-width: 5px 0 5px 5px;
    border-left-color: #ccc;
    margin-top: 5px;
    margin-right: -10px;
}

.dropdown-submenu:hover>a:after {
    border-left-color: #fff;
}

.dropdown-submenu.pull-left {
    float: none;
}

.dropdown-submenu.pull-left>.dropdown-menu {
    left: -100%;
    margin-left: 10px;
    -webkit-border-radius: 6px 0 6px 6px;
    -moz-border-radius: 6px 0 6px 6px;
    border-radius: 6px 0 6px 6px;
}

HTML示例是

<div class="navbar navbar-default navbar-fixed-top" role="navigation">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">NavBar</a>
        </div>
        <div class="collapse navbar-collapse">
            <ul class="nav navbar-nav navbar-right">
                <li><a href="https://github.com/fontenele/bootstrap-navbar-dropdowns" target="_blank">GitHub Project</a></li>
            </ul>
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">Home</a></li>
                <li>
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Menu 1 <b class="caret"></b></a>
                    <ul class="dropdown-menu multi-level">
                        <li><a href="#">Action</a></li>
                        <li><a href="#">Another action</a></li>
                        <li><a href="#">Something else here</a></li>
                        <li class="divider"></li>
                        <li><a href="#">Separated link</a></li>
                        <li class="divider"></li>
                        <li><a href="#">One more separated link</a></li>
                        <li class="dropdown-submenu">
                            <a href="#" class="dropdown-toggle" data-toggle="dropdown">Go to level 2</a>
                            <ul class="dropdown-menu">
                                <li><a href="#">Action</a></li>
                                <li class="dropdown-submenu">
                                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Go to level 3</a>
                                    <ul class="dropdown-menu">
                                        <li class="dropdown-submenu">
                                            <a href="#" class="dropdown-toggle" data-toggle="dropdown">Go to level 4</a>
                                            <ul class="dropdown-menu">
                                                <li><a href="#">Action</a></li>
                                                <li><a href="#">Another action</a></li>
                                                <li><a href="#">Something else here</a></li>
                                                <li class="divider"></li>
                                                <li><a href="#">Separated link</a></li>
                                                <li class="divider"></li>
                                                <li><a href="#">One more separated link</a></li>
                                            </ul>
                                        </li>
                                    </ul>
                                </li>
                            </ul>
                        </li>
                    </ul>
                </li>
                <li>
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Menu 2 <b class="caret"></b></a>
                    <ul class="dropdown-menu">
                        <li><a href="#">Action</a></li>
                        <li><a href="#">Another action</a></li>
                        <li><a href="#">Something else here</a></li>
                        <li class="divider"></li>
                        <li><a href="#">Separated link</a></li>
                        <li class="divider"></li>
                        <li><a href="#">One more separated link</a></li>
                        <li class="dropdown-submenu">
                            <a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown</a>
                            <ul class="dropdown-menu">
                                <li><a href="#">Action</a></li>
                                <li><a href="#">Another action</a></li>
                                <li><a href="#">Something else here</a></li>
                                <li class="divider"></li>
                                <li><a href="#">Separated link</a></li>
                                <li class="divider"></li>
                                <li class="dropdown-submenu">
                                    <a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown</a>
                                    <ul class="dropdown-menu">
                                        <li class="dropdown-submenu">
                                            <a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown</a>
                                            <ul class="dropdown-menu">
                                                <li><a href="#">Action</a></li>
                                                <li><a href="#">Another action</a></li>
                                                <li><a href="#">Something else here</a></li>
                                                <li class="divider"></li>
                                                <li><a href="#">Separated link</a></li>
                                                <li class="divider"></li>
                                                <li><a href="#">One more separated link</a></li>
                                            </ul>
                                        </li>
                                    </ul>
                                </li>
                            </ul>
                        </li>
                    </ul>
                </li>
            </ul>
        </div><!--/.nav-collapse -->
    </div>
</div>

<div class="container">
    <div class="navbar-template text-center">
        <h1>Bootstrap NavBar</h1>
        <p class="lead text-info">NavBar with too many childs.</p>
        <a target="_blank" href="https://bootsnipp.com/snippets/featured/multi-level-dropdown-menu-bs3">Thanks to msurguy (Multi level dropdown menu BS3)</a>
    </div>
</div>

“不起作用”是指我可以看到显示“转到第2级”的选项，但是当我将鼠标悬停在“转到第2级”上时，第2级的内容不会扩展/显示选项。

您知道我如何使此级别正常工作吗？ 假设代码应该在我的环境中工作，那么问题可能出在Core2.1，Bootstrap还是浏览器问题上？

谢谢