Project Euler problem 20 is another problem that would be difficult if tackled as the writer intended it to be but is trivial with the use of BigInteger. One is asked to report the sum of the digits of 100!, which can either be done with tricky byte array manipulation or by directly computing 100! as a BigInteger, converting that to a string, and then parsing each character and adding the result to a running sum. It's late, so I did the latter.

using System;
using System.Numerics;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Problem020
    class Program
        static void Main(string[] args)
            BigInteger hugeFactorial = new BigInteger(1);
            int digitsSum = 0;

            for (int i = 1; i <= 100; i++)
                hugeFactorial *= i;

            string grosNombre = hugeFactorial.ToString();

            foreach (char c in grosNombre)
                digitsSum += Int32.Parse(c.ToString());