// Program.cs

app.UseAuthorization();

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.MapFallback(async (ctx) =>
{
    ctx.Response.Body.Write(Encoding.UTF8.GetBytes("404 from Fallback"));
});

app.Run();

// HomeController.cs

[Route("{*url}", Order = 9999)]
public IActionResult Page404()
{
    return View();
}

// Page404.cshtml
@{
    ViewData["Title"] = "404";
}

<div>404 for {*url}</div>

[Route("/test404")]
public IActionResult test404()
{
    // 一些業(yè)務(wù)處理，最終返回404 
    // return NotFound();
    return StatusCode(404);
}

app.Use((context, next) =>
{
    var res = next(context);
    if (context.Response.StatusCode == 404)
    {
        context.Response.StatusCode = 200;
        context.Response.Body.Write(Encoding.UTF8.GetBytes("404 from Middleware"));
    }

    return res;
});

app.UseStatusCodePagesWithReExecute("/error/{0}");

// HomeController.cs
[Route("test401")]
public IActionResult test401()
{
    return StatusCode(401);
}

  public class ErrorController : Controller
  {
      [Route("error/404", Order = 9)]
      public IActionResult Error404()
      {
          ViewBag.code = 404;
          return View();
      }

      [Route("error/{code:int}", Order = 1)]
      public IActionResult Error(int code)
      {
          ViewBag.code = code;
          switch (code)
          {
              case 404:
                  ViewBag.msg = "對不起，請求的資源不存在。";
                  break;
              case 401:
                  ViewBag.msg = "對不起，您無權(quán)限訪問此頁面。";
                  break;
              default:
                  ViewBag.msg = "服務(wù)異常，請稍后重試！";
                  break;
          }

          return View("Error404");
      }
  }

// Error404.cshtml
@{
}

<div>@ViewBag.code : @ViewBag.msg</div>