"Even Fibonacci numbers" ( https://projecteuler.net/problem=2 )

ProjectEuler-0002.cs

// Also available as a Gist @ http://csharppad.com/gist/b9aed1a7b46d1c951f73e18032f0df84
using System;
using System.Collections.Generic;
using System.Linq;
using System.Numerics;

namespace ProjectEuler
{
    class Program
    {
        static void Main(string[] args)
        {
            int maxPos = 0;
            int currentPos = 0;
            BigInteger maxValue = 0;
            BigInteger sum = 0;

            // A function that checks to see if we're done yet.
            Func<BigInteger, bool> isDone = x => x > 4000000;

            // Keep looping until we've found a Fibonacci sequence value that is more than 4 million.
            // The Fibonacci position/value we care about is the one -IMMEDIATELY BEFORE- the one with a value over 4 million.
            while (true)
            {
                currentPos++;
                var valueAtPos = Fibonacci.GetValueByPosition(currentPos);

                if (valueAtPos > 4000000)
                    break;

                Console.WriteLine(String.Format("Position {0} = {1}", currentPos, valueAtPos));
                sum += valueAtPos;
                maxValue = valueAtPos;
                maxPos = currentPos;
            }

            // Find all of the Fibonacci values from 1..maxPos that have even values, then calculate their sum.
            var evenKeyFibonacciValues = Enumerable.Range(1, maxPos).Select(x => Fibonacci.GetValueByPosition(x)).Where(x => x % 2 == 0);
            sum = evenKeyFibonacciValues.Aggregate((currentSum, item) => currentSum + item);
            Console.WriteLine(string.Format("Total = {0}", sum));
        }
    }

    public static class Fibonacci
    {
        public static Dictionary<int, BigInteger> Sequence;

        static Fibonacci()
        {
            Sequence = new Dictionary<int, BigInteger> {{ 1, 1 }, { 2, 2 }};
        }

        public static BigInteger GetValueByPosition(int position)
        {
            if (position <= 0)
                throw new NotSupportedException("Only positive integer values are supported in working with the Fibonacci sequence");

            // If we've previously calculated the Fibonacci term at the specified position, get it.
            if (Sequence.ContainsKey(position))
                return Sequence[position];

            // The Fibonacci term has -NOT- yet been calculated for the position; calculate it using recursion.
            var fibonacciTermValue = GetValueByPosition(position - 1) + GetValueByPosition(position - 2);

            // Add the newly-calculated term to the dictionary of known terms.
            Sequence.Add(position, fibonacciTermValue);

            // Return the new value as output.
            return fibonacciTermValue;
        }
    }
}