using System;

using System.Collections.Generic;

using System.Linq;

​

namespace Example

{

    class Foo

    {

        public DateTime Date { get; set; }

        public double Estimated { get; set; }

    }

    static class ExtensionMethods

    {

        public static IEnumerable<TOut> Accumulate<TIn, TOut>(this IEnumerable<TIn> source, Func<TIn,double> getFunction, Func<TIn,double,TOut> createFunction)

        {

            double accumulator = 0;

            foreach (var item in source)

            {

                accumulator += getFunction(item);

                yield return createFunction(item, accumulator);

            }

        }

    }

    public class Program

    {

        public static void Main()

        {

            var list = new List<Foo>

            {

                new Foo { Date = new DateTime(2018,1,1), Estimated = 1 },

                new Foo { Date = new DateTime(2018,1,2), Estimated = 2 },

                new Foo { Date = new DateTime(2018,1,3), Estimated = 3 },

                new Foo { Date = new DateTime(2018,1,4), Estimated = 4 },

                new Foo { Date = new DateTime(2018,1,5), Estimated = 5 }

            };

            var accumulatedList = list.Accumulate

            ( 

                (item)      => item.Estimated,                    //Given an item, get the value to be summed

                (item, sum) => new { Item = item, Sum = sum }     //Given an item and the sum, create an output element

            );

            foreach (var item in accumulatedList)

            {

                Console.WriteLine("{0:yyyy-MM-dd} {1}", item.Item.Date, item.Sum);

            }

​

        }

    }

}

​