C# How to find factorial of 999

In C# interview, you may be asked How To find the factorial of 999 in C#, you cannot use int, long, or even decimal because factorial values grow extremely fast.

Use BigInteger from System.Numerics.

Example:

using System;
using System.Numerics;

class Program
{
    static BigInteger Factorial(int n)
    {
        BigInteger result = 1;

        for (int i = 2; i <= n; i++)
        {
            result *= i;
        }

        return result;
    }

    static void Main()
    {
        int number = 999;

        BigInteger factorial = Factorial(number);

        Console.WriteLine(factorial);
    }
}

Output

This prints the exact value of 999! (a huge number).

Why BigInteger?

Look at the approximate limits of different data types:

• int → up to ~2 billion → only enough for 12!
• long → up to ~9 quintillion → only enough for 20!
• BigInteger → arbitrary precision (limited mainly by memory)

Alternative Solution: Short version using LINQ

using System;
using System.Linq;
using System.Numerics;

BigInteger factorial =
    Enumerable.Range(1, 999)
              .Aggregate(BigInteger.One, (a, b) => a * b);

Console.WriteLine(factorial);

Count digits of 999!

Console.WriteLine(factorial.ToString().Length);

999! contains 2,565 digits.