The largest number representable in .NET for UInt64, Decimal, and Double are about 1.84E19, 3.96E28, and 1.80E308 respectively. The factorial of 28 is already 3.05E29, which is bigger than what any number type (except the Double) can represent in .NET. The Double type has its own problems too. Beyond 4.50E15, integers can't be represented accurately anymore since a Double only has a maximum of 52 bits to represent an integer with. You might as well as craft a method to look up the answer in a pre-made dictionary or table of some sort. I don't know what your needs are or if I'm completely wrong, so feel free to correct me.
So the creationist says: Everything must have a designer. God designed everything. I say: Why is God the only exception? Why not make the "designs" (like man) exceptions and make God a creation of man?