MapBetween

Program.cs

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

namespace ConsoleApplication1
{
  public static class EnumerableEx
  {
    public static IEnumerable<TResult> MapBetween<TValue, TResult>(this IEnumerable<TValue> source, Func<TValue, TValue, TResult> f)
    {
      return from a in source.Select((x, i) => new { X = x, Index = i })
             join b in source.Skip(1).Select((x, i) => new { X = x, Index = i }) on a.Index equals b.Index
             select f(a.X, b.X);
    }
  }

  public class Program
  {
    static void Main(string[] args)
    {
      var q = new[] { 1, 2, 3, 4, 5 }.MapBetween((a, b) => a + b);
      foreach (var n in q)
      {
        Console.WriteLine(n);
      }
    }

  }
}