I would like to know how people are going about validating collections in WPF. Lets say for example that I have an observable collection of ViewModels that I am binding to the items source of a grid, and the user can add new rows to the grid and needs to fill them.

First of all I need to validate on each row to ensure that required fields of each ViewModel are filled in. This is fine and simple to do for each row.

However, the second level of validation is on the collection as a whole. For example i want to ensure that no two rows of the collection have the same identifier, or that no two rows have the same name. Im basically checking for duplicate properties within different rows. I also have more complex conditions where I must ensure that there is at least one item within the collection that has some property set.

How do I get a validation rule that would allow me to check these rules, validating on the whole collection rather than the individual items. I also want to print any validation error above the datagrid so that the user can fix the problem and the message will update or disappear as the user fixes each different rule.

Anyone have any experience of the proper way to do this?

Thanks Chris