Question

using System.Numerics;

double doubleNumber = Math.Pow(1000, 99); // = 1.0E+297
BigInteger bigBase = (BigInteger)bigNumber; // = 1000000000000000017652801462756379714374878780719864776839443139119744823869255243069012222883470359078822072829219411228534934402712624705615450492327979456500795456339201761949451160807447294527656222743617592048849967890105831362861792425329827928397252374398383022243308510390698430058459037696

Why is the BigInteger value not just 1 kagillion (1,000 followed by a bagillion 0’s)?

Answer 1

The implementation of BigInteger uses a lot of bit shifting, so when you convert a double to a BigInteger, the double representation has a finite set of bits (64 bits in for a double), so not all the bits are there to accurately represent the value.

Instead of using Math.Pow, you should be using

var bigBase = BigInteger.Pow(1000, 99);

Also, explicitly casting a numeric type to a BigInteger is the same as using the BigInteger constructor:

 var bigBase = (BigInteger)doubleNumber;

Is equivalent to:

var bigBase = new BigInteger(doubleNumber);