RePost: ASP.NET MVC Core & MVC - model validation - adding custom validation

https://docs.microsoft.com/en-us/aspnet/core/mvc/models/validation?view=aspnetcore-2.1

http://bradwilson.typepad.com/blog/2010/10/mvc3-unobtrusive-validation.html

https://blogs.msdn.microsoft.com/mvpawardprogram/2017/01/03/asp-net-core-mvc/

https://thewayofcode.wordpress.com/2012/01/18/custom-unobtrusive-jquery-validation-with-data-annotations-in-mvc-3/