Untitled

<head>
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script src="~/js/jquery.pjax.js"></script>
<script type="text/javascript">
    $(function () {
        // pjax
        $('ul a').pjax('#main')
    })
</script>
</head>

<body>
<ul>
  <li><a href="/" class="current">MAIN PAGE</a></li>
  <li><a href="/Home/About">About</a></li>
</ul>

<div id="main">@RenderBody()</div>
</body>

@{
    if (ViewBag.PJAX==true)
    {
        Layout = "~/Views/Shared/_Layout.cshtml";
    }
    else
    {
        Layout = "~/Views/Shared/_Layout.cshtml";
    }
}

public IActionResult Index()
    {
        if (string.IsNullOrEmpty(Request.Headers["X-PJAX"]))
            ViewBag.PJAX = false;
        else
            ViewBag.PJAX = true;
        return View();
    }