Basic SaveChanges() override for setting existing CreatedBy/CreatedUtc/LastModifiedBy/LastModifiedUtc columns on EF tables.

ExampleEntities.cs

public partial class ExampleEntities {
    private delegate void ColumnValueSetter(DbPropertyEntry entry, DateTimeOffset now);

    private static readonly Dictionary<string, ColumnValueSetter> ControlledColumns = new Dictionary<string, ColumnValueSetter> {
        {"CreatedBy", ColumnValueSetters.CreatedBy}
        , {"CreatedUtc", ColumnValueSetters.CreatedUtc}
        , {"LastModifiedBy", ColumnValueSetters.LastModifiedBy}
        , {"LastModifiedUtc", ColumnValueSetters.LastModifiedUtc}
    };

    public override int SaveChanges() {
        var modifiedEntries = ChangeTracker.Entries().Where(x => x.State == EntityState.Added || x.State == EntityState.Modified);
        var dateTimeOffsetUtcNow = DateTimeOffset.UtcNow;
        foreach (var entry in modifiedEntries) {
            foreach (var columnName in entry.CurrentValues.PropertyNames.Intersect(ControlledColumns.Keys)) {
                ControlledColumns[columnName]((DbPropertyEntry)entry.Member(columnName), dateTimeOffsetUtcNow);
            }
        }
        return base.SaveChanges();
    }

    public int SaveChangesWithoutOverridingAuditColumnValues() {
        return base.SaveChanges();
    }

    private static class ColumnValueSetters {
        private static readonly Func<object, ColumnValueSetter> GetColumnValueSetterForRecordInsert = initialValue => {
            return (entry, now) => {
                entry.CurrentValue = (entry.EntityEntry.State == EntityState.Added ? null : entry.OriginalValue) ?? initialValue;
            };
        };

        public static readonly ColumnValueSetter CreatedBy = delegate (DbPropertyEntry entry, DateTimeOffset now) {
            var columnValueSetter = GetColumnValueSetterForRecordInsert(Thread.CurrentPrincipal.Identity.Name);
            columnValueSetter(entry, now);
        };

        public static readonly ColumnValueSetter CreatedUtc = delegate (DbPropertyEntry entry, DateTimeOffset now) {
            var columnValueSetterForRecordInsert = GetColumnValueSetterForRecordInsert(now);
            columnValueSetterForRecordInsert(entry, now);
        };

        public static readonly ColumnValueSetter LastModifiedBy = (entry, now) => entry.CurrentValue = Thread.CurrentPrincipal.Identity.Name;

        public static readonly ColumnValueSetter LastModifiedUtc = (entry, now) => entry.CurrentValue = now;
    }

}