Checking login user role in razor page

@if (Request.IsAuthenticated && User.Identity.Name=="administrator")
{
     <div id="sidebar">
        <div class="module">
        <ul class="menu">
                            <li>@Html.ActionLink("Home", "Index", "Home")</li>
                            <li>@Html.ActionLink("About", "About", "Home")</li>
                            <li>@Html.ActionLink("Contact", "Contact", "Home")</li>
                        </ul>
         </div>
         <div class="mainContent">
             Hello, @User.Identity.Name !
         </div>
     </div>

This is my layout if the user is authenticated as administrator but this sort of check looks no good, I need to check the role of the user not his name.

Here is the controler method

    public ActionResult AuthenticatedUserLayout(string username) 
    {
        var lst=userContext.UserProfiles.ToList();
        var user = lst.Select(u => u.UserName == username);

        if(IsAdmin(Session["LoginUser"].ToString())) return View(user); else return Index();
    }

I also find that return View(user) is no good, because I don't know how to make any use of that user.

@if (Request.IsAuthenticated && User.IsInRole("Administrators"))
{
     <div id="sidebar">
        <div class="module">
           <ul class="menu">
              <li>@Html.ActionLink("Home", "Index", "Home")</li>
              <li>@Html.ActionLink("About", "About", "Home")</li>
              <li>@Html.ActionLink("Contact", "Contact", "Home")</li>
            </ul>
         </div>
         <div class="mainContent">
             Hello, @User.Identity.Name !
         </div>
     </div>
}

For ASP.NET Core Razor Pages

if (User.Identity.IsAuthenticated && User.IsInRole("Administrator"))

Dave's answer is correct. I would suggest that you consider using a property on your model called IsAdministrator or CanSeeSidebar and treat answering that question as domain logic.

The view should work only with the model. Looking at the thread, reading from a database, are the same in respect that they answer domain questions. All those types of questions should be answered before your controller hands the model off to the view.