http://ayende.com/Blog/archive/2007/10/21/The-IoC-mind-set-Validation.aspx

But there are situations when it is better to show a field-level error when a database error occurs

As far as database errors goes, the user does not care about those. There is nothing user can do. In that case a system error should be displayed, through UI general error handling system.

We were talking at Kaizenconf about advanced uses of IOC containers and vailidation came up. Ayende has an old blog entry about it, sorry I don’t have the link handy, but you might want to search for it.

I like what Michael posted in the comments at http://mokhan.ca/blog/CommentView,guid,36584ca0-a20f-4df4-acdc-c844d15c0b2f.aspx#commentstart

He suggested a fluent interface for validation…

CreateRule.For()
.Property(u => u.FirstName)
.WithRule((u, value) => !value.IsNullOrEmpty())
.WithError(“First name is required”);

This would allow me to validate a DTO instead of a domain object, for messages to return to the UI, and that would allow me to ensure the domain does not enter an invalid state. So user.change_first_name_to(first_name); would throw if it was invalid. However, this might violate DRY.

@Maxim
As far the database validation goes, I suppose if we were to ensure that our domain object do not enter an invalid state that would reduce our chances for allowing bad data into the db. Our layers of protection, would be js validation in the ui, plus dto validation in service, and throw exception from the domain if it gets that far.

I think I’m ranting… but perhaps this suggests that I could use some more guidance in “validation”, or at least a working example!

To display notifications (validation/other errors) I use age old notification pattern, which simply is a collection of messages to display .

I’ve also had my take on validation “framework” with empathis on fluent construction of validation rules which looked good enough for C# 2.0 Haven’t had a chance to come back to it with C# 3.0 yet

the one you’ve mentioned validates only objects, but sometimes you can’t create object because of incorrect user input and sometimes an error occur during saving object to the database for example. And all these issues should be reflected in UI in the same way.