A custom validation attribute allows you to create metadata, that you can use in the data model to validate data fields. In order to validate when a collection is null or empty, you may create a custom validation attribute, which returns false in case the object is null or empty. [crayon-56d4b8aa38fbd868986865/] You can now decorate classes using this attribute, i.e.: [crayon-56d4b8aa38ff3854591558/] Let’s assume that you have a WebAPI method test/validation, which:...