Using DataAnnotations with Entity Framework

Question

I have used the Entity Framework with VS2010 to create a simple person class with properties, firstName, lastName, and email. If I want to attach DataAnnotations like as is done in this blog post I have a small problem because my person class is dynamically generated. I could edit the dynamically generated code directly but any time I have to update my model all my validation code would get wiped out.

First instinct was to create a partial class and try to attach annotations but it complains that I'm trying to redefine the property. I'm not sure if you can make property declarations in C# like function declarations in C++. If you could that might be the answer. Here's a snippet of what I tried:

namespace PersonWeb.Models
{
  public partial class Person
  {
    [RegularExpression(@"(\w|\.)+@(\w|\.)+", ErrorMessage = "Email is invalid")]
    public string Email { get; set; } 
    /* ERROR: The type 'Person' already contains a definition for 'Email' */
  }
}

Answer 1

You need to either use a metadata "buddy" class or (my preference) project onto a presentation model instead of binding views directly to entities.

Answer 2

A buddy class is more or less the direction your code snippet is journeying, except your manually coded partial Person class would have an inner class, like:

[MetadataType(typeof(Person.Metadata))]
public partial class Person {
    private sealed class MetaData {
        [RegularExpression(...)]
        public string Email { get; set; }
    }
}

Or you could have your manually partial Person class and a separate Meta class like:

[MetadataType(typeof(PersonMetaData))]
public partial class Person { }

public class PersonMetaData {
[RegularExpression(...)]
public string Email;
}

These are workarounds and having a mapped Presentation class may be more suitable.