How to return 401 instead of 302 in ASP.NET Core?

As of ASP.NET Core 2.x:

services.ConfigureApplicationCookie(options =>
{
    options.Events.OnRedirectToLogin = context =>
    {
        context.Response.StatusCode = 401;    
        return Task.CompletedTask;
    };
});

If the request header contains X-Requested-With: XMLHttpRequest the status code will be 401 instead of 302

private static bool IsAjaxRequest(HttpRequest request)
    {
        return string.Equals(request.Query["X-Requested-With"], "XMLHttpRequest", StringComparison.Ordinal) ||
            string.Equals(request.Headers["X-Requested-With"], "XMLHttpRequest", StringComparison.Ordinal);
    }

See on gitHub: https://github.com/aspnet/Security/blob/5de25bb11cfb2bf60d05ea2be36e80d86b38d18b/src/Microsoft.AspNetCore.Authentication.Cookies/Events/CookieAuthenticationEvents.cs#L40-L52

For asp.net mvc core USE THIS INSTEAD

 services.ConfigureApplicationCookie(options =>
        {
            options.LoginPath = new PathString("/Account/Login");
            options.LogoutPath = new PathString("/Account/Logout");

            options.Events.OnRedirectToLogin = context =>
            {
                if (context.Request.Path.StartsWithSegments("/api")
                    && context.Response.StatusCode == StatusCodes.Status200OK)
                {
                    context.Response.Clear();
                    context.Response.StatusCode = StatusCodes.Status401Unauthorized;
                    return Task.CompletedTask;
                }
                context.Response.Redirect(context.RedirectUri);
                return Task.CompletedTask;
            };
        });

services.Configure<IdentityOptions>(options =>
{
   options.Cookies.ApplicationCookie.LoginPath = new PathString("/");
   options.Cookies.ApplicationCookie.Events = new CookieAuthenticationEvents()
   {
      OnRedirectToLogin = context =>
      {
         if (context.Request.Path.Value.StartsWith("/api"))
         {
            context.Response.Clear();
            context.Response.StatusCode = 401;
            return Task.FromResult(0);
         }
         context.Response.Redirect(context.RedirectUri);
         return Task.FromResult(0);
      }
   };
});

Source:

https://www.illucit.com/blog/2016/04/asp-net-5-identity-302-redirect-vs-401-unauthorized-for-api-ajax-requests/