Surprisingly, although ASP.NET Core provides plenty of options for handling 404 errors gracefully, the ASP.NET Core MVC runtime doesn’t take advantage of them by default.

Learn to change the default behavior and return HTTP 404 when action methods in ASP.NET Core return null values.

Here's everything you need to write code for the Session object in ASP.NET Core -- including why you can't expect to migrate your existing ASP.NET MVC application to ASP.NET MVC Core (though Peter has ...