SumBy

Returns the sum of a collection, after mapping each element to a value using the provided function.

• Use IEnumerable.Select() to map each element of either collection to a double, IEnumerable.Sum() to sum the values.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static double SumBy<T>(IEnumerable<T> data, Func<T,double> map)
  {
    return data.Select(map).Sum();
  }
}

var p = new[] {
  new { a = 3, b = 2},
  new { a = 2, b = 1}
};

_30s.SumBy(p, x => x.a); // 5