A value larger than ULong? Computing 100!

Question

I'm trying to compute 100! and there doesn't seem to be a built-in factorial function. So, I've written:

Protected Sub ComputeFactorial(ByVal n As ULong)
        Dim factorial As ULong = 1
        Dim i As Integer
        For i = 1 To n
            factorial = factorial * i
        Next
        lblAnswer.Text = factorial
    End Sub

Unfortunately, running this with the value of 100 for n rseults in

Value was either too large or too small for a UInt64.

So, is there a larger data type for holding numbers? Am i mistaken in my methods? Am I helpless?

Answer 1

Sounds like Project Euler.

.NET 4.0 has System.Numerics.BigInteger, or you can pick up a pretty sweet implementation here:
C# BigInteger Class

Edit: treed :(

I'll add - the version at CodeProject has additional features like integer square root, a primality test, Lucas sequence generation. Also, you don't have direct access to the buffer in the .NET implementation which was annoying for a couple things I was trying.

Answer 2

Until you can use System.Numerics.BigInteger you are going to be stuck using a non-Microsoft implementation like BigInteger on Code Project.

Answer 3

Hint: use an array to store the digits of the number. You can tell by inspection that the result will not have more than 200 digits.