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

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.

By using a BigInteger class; there's one in the the J# libraries (definitely accessible from C#), another in F# (need to test this one), and there are freestanding implementations such as this one in pure C#.

What is it that you wish to use these numbers for? If you are doing calculations with really big numbers, do you still need the accuracy down to the last digit? If not, you should consider using floating point values instead. They can be huge, the max value for the double type is 1.79769313486231570E+308, (in case you are not used to scientific notation it means 1.79769313486231570 multiplied by 10000000...0000 - 308 zeros).

That should be large enough for most applications