Keep a history of values for specific properties of EF entities

I have a requirement to keep a history of values of some fields in an EF4 ASP.NET MVC3 application. This just needs to be a log file of sorts, log the user, datetime, tablename, fieldname, oldvalue, newvalue.

Although it would be pretty easy to code this in various save routines, I'm wondering if I can get global coverage by wiring it into some sort of dataannotation, so that I can perhaps declare

[KeepHistory()]
public string Surname { get; set; } 

in my partial class (I'm using POCO but generated from a T4 template).

So Questions

1) Is this a bad idea ? I'm basically proposing to side-effect changes to an entity, not directly referenced by the code, as a result of an annotation.

2) Can it be done ? Will I have access to the right context to tie up with my unit of work so that changes get saved or dropped with the context save?

3) Is there a better way?

4) If you suggest I do this, any pointers would be appreciated - everything I've read is for validation, which may not be the best starting poi开发者_运维技巧nt.

Actually, validation might be a good starting point. Since an attribute does not know about which property or class it was assigned to, but a validation-attribute gets called by the validation framework with all the necessary informátion. If you implement the System.ComponentModel.DataAnnotations.ValidationAttribute class you can override the IsValid(object, ValidationContext) method, which gives you the actual value of the property, the name of the property and the container.

This might take a lot of work, since you need to get to the currently logged-in user etc. I'm guessing that the .NET implementation provides some sort of caching for the specific attributes on an entity type, which would be a pain to implement by yourself.

Another way, would be to use the ObjectStateManager exposed by your EF ObjectContext, which can provide you with the ObjectStateEntry-objects for all entities of a given state. See the ObjectStateManager.GetObjectStateEntries(EntityState) method, for more information about how to call it (and when). The ObjectStateEntry actually contains a record of the original and current-values, which can be compared to find any changes made within the lifetime of the current ObjectContext.

You might consider using the ObjectStateManager to inject your custom logging behavior, while this behavior decides based on property-attributes which change should be logged.