According to the reference source it appears that it stores it's information as a uint[]
for the data and a int for the sign.
namespace System.Numerics
{
/// <summary>Represents an arbitrarily large signed integer.</summary>
[Serializable]
public struct BigInteger : IFormattable, IComparable, IComparable<BigInteger>, IEquatable<BigInteger>
{
// For values int.MinValue < n <= int.MaxValue, the value is stored in sign
// and _bits is null. For all other values, sign is +1 or -1 and the bits are in _bits
internal int _sign;
internal uint[] _bits;
Here is the code it executes to perform ToByteArray()
// Return the value of this BigInteger as a little-endian twos-complement
// byte array, using the fewest number of bytes possible. If the value is zero,
// return an array of one byte whose element is 0x00.
public byte[] ToByteArray() {
if (_bits == null && _sign == 0)
return new byte[] { 0 };
// We could probably make this more efficient by eliminating one of the passes.
// The current code does one pass for uint array -> byte array conversion,
// and then another pass to remove unneeded bytes at the top of the array.
uint[] dwords;
byte highByte;
if (_bits == null) {
dwords = new uint[] { (uint)_sign };
highByte = (byte)((_sign < 0) ? 0xff : 0x00);
}
else if(_sign == -1) {
dwords = (uint[])_bits.Clone();
NumericsHelpers.DangerousMakeTwosComplement(dwords); // mutates dwords
highByte = 0xff;
} else {
dwords = _bits;
highByte = 0x00;
}
byte[] bytes = new byte[checked(4 * dwords.Length)];
int curByte = 0;
uint dword;
for (int i = 0; i < dwords.Length; i++) {
dword = dwords[i];
for (int j = 0; j < 4; j++) {
bytes[curByte++] = (byte)(dword & 0xff);
dword >>= 8;
}
}
// find highest significant byte
int msb;
for (msb = bytes.Length - 1; msb > 0; msb--) {
if (bytes[msb] != highByte) break;
}
// ensure high bit is 0 if positive, 1 if negative
bool needExtraByte = (bytes[msb] & 0x80) != (highByte & 0x80);
byte[] trimmedBytes = new byte[msb + 1 + (needExtraByte ? 1 : 0)];
Array.Copy(bytes, trimmedBytes, msb + 1);
if (needExtraByte) trimmedBytes[trimmedBytes.Length - 1] = highByte;
return trimmedBytes;
}
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