Entity Framework 4.1 DbContext Override SaveChanges to Audit Property Change

Question

I am trying to implement a constrained 'audit log' of property changes to a properties in a set of classes. I have successfully found out how to set CreatedOn|ModifiedOn type properties, but am failing to find out how to 'find' the property that has been modified.

Example:

public class TestContext : DbContext
{
    public override int SaveChanges()
    {
        var utcNowAuditDate = DateTime.UtcNow;
        var changeSet = ChangeTracker.Entries<IAuditable>();
        if (changeSet != null)
            foreach (DbEntityEntry<IAuditable> dbEntityEntry in changeSet)
            {

                switch (dbEntityEntry.State)
                {
                    case EntityState.Added:
                        dbEntityEntry.Entity.CreatedOn = utcNowAuditDate;
                        dbEntityEntry.Entity.ModifiedOn = utcNowAuditDate;
                        break;
                    case EntityState.Modified:
                        dbEntityEntry.Entity.ModifiedOn = utcNowAuditDate;
                        //some way to access the name and value of property that changed here
                        var changedThing = SomeMethodHere(dbEntityEntry);
                        Log.WriteAudit("Entry: {0} Origianl :{1} New: {2}", changedThing.Name,
                                        changedThing.OrigianlValue, changedThing.NewValue)
                        break;
                }
            }
        return base.SaveChanges();
    }
}

So, is there a way to access the property that changed with this level of detail in EF 4.1 DbContext?

Answer 1

Very, very rough idea:

foreach (var property in dbEntityEntry.Entity.GetType().GetProperties())
{
    DbPropertyEntry propertyEntry = dbEntityEntry.Property(property.Name);
    if (propertyEntry.IsModified)
    {
        Log.WriteAudit("Entry: {0} Original :{1} New: {2}", property.Name,
            propertyEntry.OriginalValue, propertyEntry.CurrentValue);
    }
}

I have no clue if this would really work in detail, but this is something I would try as a first step. Of course there could be more then one property which has changed, therefore the loop and perhaps multiple calls of WriteAudit.

The reflection stuff inside of SaveChanges could become a performance nightmare though.

Edit

Perhaps it is better to access the underlying ObjectContext. Then something like this is possible:

public class TestContext : DbContext
{
    public override int SaveChanges()
    {
        ChangeTracker.DetectChanges(); // Important!

        ObjectContext ctx = ((IObjectContextAdapter)this).ObjectContext;

        List<ObjectStateEntry> objectStateEntryList =
            ctx.ObjectStateManager.GetObjectStateEntries(EntityState.Added
                                                       | EntityState.Modified 
                                                       | EntityState.Deleted)
            .ToList();

       foreach (ObjectStateEntry entry in objectStateEntryList)
       {
           if (!entry.IsRelationship)
           {
               switch (entry.State)
               {
                   case EntityState.Added:
                       // write log...
                       break;
                   case EntityState.Deleted:
                       // write log...
                       break;
                   case EntityState.Modified:
                   {
                       foreach (string propertyName in
                                    entry.GetModifiedProperties())
                       {
                           DbDataRecord original = entry.OriginalValues;
                           string oldValue = original.GetValue(
                               original.GetOrdinal(propertyName))
                               .ToString();

                           CurrentValueRecord current = entry.CurrentValues;
                           string newValue = current.GetValue(
                               current.GetOrdinal(propertyName))
                               .ToString();

                           if (oldValue != newValue) // probably not necessary
                           {
                               Log.WriteAudit(
                                   "Entry: {0} Original :{1} New: {2}",
                                   entry.Entity.GetType().Name,
                                   oldValue, newValue);
                           }
                       }
                       break;
                   }
               }
           }
       }
       return base.SaveChanges();
    }
}

I've used this myself in EF 4.0. I cannot find a corresponding method to GetModifiedProperties (which is the key to avoid the reflection code) in the DbContext API.

Edit 2

Important: When working with POCO entities the code above needs to call DbContext.ChangeTracker.DetectChanges() at the beginning. The reason is that base.SaveChanges is called too late here (at the end of the method). base.SaveChanges calls DetectChanges internally, but because we want to analyze and log the changes before, we must call DetectChanges manually so that EF can find all modified properties and set the states in the change tracker correctly.

There are possible situations where the code can work without calling DetectChanges, for example if DbContext/DbSet methods like Add or Remove are used after the last property modifications are made since these methods also call DetectChanges internally. But if for instance an entity is just loaded from DB, a few properties are changed and then this derived SaveChanges is called, automatic change detection would not happen before base.SaveChanges, finally resulting in missing log entries for modified properties.

I've updated the code above accordingly.