.net - Calculate factorials in C#

Question

How can you calculate large factorials using C#? Windows calculator in Win 7 overflows at Factorial (3500). As a programming and mathematical question I am interested in knowing how you can calculate factorial of a larger number (20000, may be) in C#. Any pointers?

[Edit] I just checked with a calc on Win 2k3, since I could recall doing a bigger factorial on Win 2k3. I was surprised by the way things worked out.

1. Calc on Win2k3 worked with even big numbers. I tried !50000 and I got an answer, 3.3473205095971448369154760940715e+213236

2. It was very fast while I did all this.

The main question here is not only to find out the appropriate data type, but also a bit mathematical. If I try to write a simple factorial code in C# [recursive or loop], the performance is really bad. It takes multiple seconds to get an answer. How is the calc in Windows 2k3 (or XP) able to perform such a huge factorial in less than 10 seconds? Is there any other way of calculating factorial programmatically in C#?

Answer 1

Have a look at the BigInteger structure:

http://msdn.microsoft.com/en-us/library/system.numerics.biginteger.aspx

Maybe this can help you implement this functionality.

CodeProject has an implementation for older versions of the framework at http://www.codeproject.com/KB/cs/biginteger.aspx.