public static string IsSelected( this HtmlHelper html, string controller = null, string action = null, string area = null ) {
    string cssClass = "active";
    string currentAction = (string)html.ViewContext.RouteData.Values[ "action" ];
    string currentController = (string)html.ViewContext.RouteData.Values[ "controller" ];
    string currentArea = html.ViewContext.RouteData.DataTokens[ "area" ] == null ? String.Empty : (string)html.ViewContext.RouteData.DataTokens[ "area" ];

    if ( String.IsNullOrEmpty( controller ) )
        controller = currentController;

    if ( String.IsNullOrEmpty( action ) )
        action = currentAction;

    if ( String.IsNullOrEmpty( area ) )
        area = currentArea;

    if ( controller.ToLower() == currentController.ToLower() && action.ToLower() == currentAction.ToLower() && area == currentArea ) {
        return cssClass;
    }
    else {
        return string.Empty;
    }
}

<li class="treeview @Html.IsSelected( controller: "mycontroller", action : string.Empty, area : string.Empty )">
    <a href="#">
        <i class="fa fa-list-alt"></i> <span>Menu 1</span>
        <span class="pull-right-container">
            <i class="fa fa-angle-right pull-right"></i>
        </span>
    </a>
    <ul class="treeview-menu">
        <li class="@Html.IsSelected( controller: "mycontroller", action : "myaction", area : string.Empty )">
            <a href="@Url.Action("myaction", "mycontroller" )">
                <i class="fa fa-retweet"></i>
                My Action &nbsp;
            </a>
        </li>
    </ul>
</li>