محاسبه فاکتوریل و سری فیبوناچی اعداد خیلی بزرگ!

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Numerics;

public class CFibonacci
{
public static IEnumerable<BigInteger> BigFib(Int64 toNumber)
{
BigInteger previous = 0;
BigInteger current = 1;

for (Int64 y = 1; y <= toNumber; y++)
{
var auxiliar = current;
current += previous;
previous = auxiliar;
yield return current;
}
}

public static IEnumerable<BigInteger> BigFact(Int64 toNumber)
{
BigInteger i = 0;
BigInteger fact = 1;
if (toNumber <= 1)
{
yield return 1;
}
else
{
for (i = 1; i <= toNumber; i++)
{
fact *= i;
}
yield return fact;
}

}
}
}

private void button1_Click(object sender, EventArgs e)
{
var num = Convert.ToInt32(txtNumber.Text);
var fib = CFibonacci.BigFib(num).Last();
txtFib.Text = fib.ToString();

var fact = CFibonacci.BigFact(num).Last();
txtFact.Text = fact.ToString();
}