根据用户角色显示或隐藏导航菜单

@if (SignInManager.IsSignedIn(User) && User.IsInRole("Admin"))
{
    <li class="nav-item">
        <a class="nav-link" asp-controller="Admin" asp-action="ListRoles">角色管理</a>
    </li>
}

默认的拒绝访问地址是 AccessDenied：
http://localhost:3290/Account/AccessDenied?ReturnUrl=%2FAdmin%2FListRoles

[HttpGet]
[AllowAnonymous]
public IActionResult AccessDenied()
{
    return View();
}

AccessDenied 页面：

@{
    ViewBag.Title = "拒绝访问";
}
<div class="text-center">
    <h1 class="text-danger">拒绝访问</h1>
    <h6 class="text-danger">您没有查看此资源的权限</h6>
    <img src="~/images/noaccess.png" style="width: 300px; height: 300px;" />
</div>

自定义拒绝访问地址

也可以在 ConfigureServices 里面自定义拒绝访问的地址：

services.ConfigureApplicationCookie(options =>
{
    options.AccessDeniedPath = "/Account/Fangyu";
});

此时访问拒绝地址就会变成 Fangyu：
http://localhost:3290/Account/Fangyu?ReturnUrl=%2FAdmin%2FListRoles