GetByPrimaryKey for LINQ

partial class AcessaBanco
    {
        public IEnumerable<T> GetByPrimaryKeys<T>(params object[] IDs) where T : class
        {
            var mapping = this.Mapping.GetTable(typeof(T));
            var pkfield = mapping.RowType.DataMembers.SingleOrDefault(d => d.IsPrimaryKey);
            if (pkfield == null) throw new Exception(string.Format("Table {0} does not contain a Primary Key field or is a composite primary key", mapping.TableName));
            var param = Expression.Parameter(typeof(T), "e");
            List<T> l = new List<T>();
            foreach (var ID in IDs)
            {
                var predicate = Expression.Lambda<Func<T, bool>>(Expression.Equal(Expression.Property(param, pkfield.Name), Expression.Constant(Convert.ChangeType(ID, pkfield.Type))), param);
                var obj = this.GetTable<T>().SingleOrDefault(predicate);
                if (obj != null) l.Add(obj);
            }
            return l.AsEnumerable();
        }

        public T GetByPrimaryKey<T>(object ID, bool CreateIfNotExists = false) where T : class
        {
            bool isnew = false;
            return this.GetByPrimaryKey<T>(ID, CreateIfNotExists, ref isnew);
        }

        public T GetByPrimaryKey<T>(object ID, bool CreateIfNotExists, ref bool IsNew) where T : class
        {
            T obj = null;
            IsNew = false;
            try { obj = this.GetByPrimaryKeys<T>(new[] { ID }).SingleOrDefault(); } catch { }
            if (obj == null && CreateIfNotExists)
            {
                IsNew = true;
                obj = Activator.CreateInstance<T>();
                this.GetTable<T>().InsertOnSubmit(obj);
            }
            return obj;
        }
    }