How to override Entity Framework validation error messages

I have an Entity Framework 4.1 model that supports multiple ASP.NET MVC web applications. I use DataAnnotations to define and localize label text and validation rules and error messages.

For some applications, I need the label text for certain fields to differ from the standard, model-defined text. This is easy to achieve for the labels themselves: I retrieve the text I need from a local resource file, associated with the view. However, the label text is also used in validation error messages such as "{fieldname} must have a maximum length of 50 characters".

What is the best way to alter the validation messages without changing the Annotations on the model classes?

You have coupled your validation to your entity models. The way to avoid this is to create view models from your entities and put the validation on to those.

Example: If you have the Entity...

public class Product
{
    public string Name {get; set;}
}

you could create two different view models:

public class ProductPageViewModel1
{
    [Required]
    public string Name {get; set;}
}

public class ProductPageViewModel2
{
    [DisplayName("Foo")]
    public string Name {get; set;}

}

Map the entity to the view model and use these in your views.