How do you deal with numbers larger than UInt64 (C#)

Question

In C#, how can one store and calculate with numbers that significantly exceed UInt64's max value (18,446,744,073,709,551,615)?

Answer 1

Can you use the .NET 4.0 beta? If so, you can use BigInteger.

Otherwise, if you're sticking within 28 digits, you can use decimal - but be aware that obviously that's going to perform decimal arithmetic, so you may need to round at various places to compensate.