Valudation for DTO and useful response in case of error

[denis-tsv]

Now CustomersController.Create method returned not useful for API clients response: one error message for all DTO properties.
If we will use ModelState then we can solve two problems:

1. We will receive useful response where each property has array or error messages
2. If DTO property is not valid (for example too long) and it is not mapped to ValueObject then we also will receive an error message.

[vkhorikov]

I don't like ModelState because it produces json that is too verbose. It's only useful when you work with Razor views or some other built-in asp.net web views. In scenarios with rich UI clients (e.g written in React), I recommend returning custom DTOs.

You can gather multiple errors with the approach I described here: https://enterprisecraftsmanship.com/posts/combining-asp-net-core-attributes-with-value-objects/