How do I define a predicate from a keySelector at runtime with anonymous?

IEnumerable<IGrouping<object, DataRow>> result = null;
            Func<DataRow, bool> predicate = null;
            switch (fields.Length)
            {
                case 2:
                    result = dt.AsEnumerable().GroupBy(x => new { A = x[fields[0]], B = x[fields[1]] });
                    predicate = x => x[fields[0]].Equals(result.FirstOrDefault().Key.A) && x[fields[1]].Equals(result.FirstOrDefault().Key.B);
                    return result;
                default://case 1
                    result = dt.AsEnumerable().GroupBy(x => x[fields[0]]);
                    predicate = x => x[fields[0]].Equals(result.FirstOrDefault().Key);
                    return result;
            }