.NET BigInteger Multiplication

public static BigInteger operator *(BigInteger left, BigInteger right)
{
    int sign = 1;
    BigIntegerBuilder bigIntegerBuilder = new BigIntegerBuilder(left, ref sign);
    BigIntegerBuilder bigIntegerBuilder2 = new BigIntegerBuilder(right, ref sign);
    bigIntegerBuilder.Mul(ref bigIntegerBuilder2);
    return bigIntegerBuilder.GetInteger(sign);
}

...

public void Mul(ref BigIntegerBuilder regMul)
{
    if (regMul._iuLast == 0)
    {
        this.Mul(regMul._uSmall);
        return;
    }
    if (this._iuLast == 0)
    {
        uint uSmall = this._uSmall;
        if (uSmall == 1u)
        {
            this = new BigIntegerBuilder(ref regMul);
            return;
        }
        if (uSmall != 0u)
        {
            this.Load(ref regMul, 1);
            this.Mul(uSmall);
            return;
        }
    }
    else
    {
        int num = this._iuLast + 1;
        this.SetSizeKeep(num + regMul._iuLast, 1);
        int num2 = num;
        while (--num2 >= 0)
        {
            uint uMul = this._rgu[num2];
            this._rgu[num2] = 0u;
            uint num3 = 0u;
            for (int i = 0; i <= regMul._iuLast; i++)
            {
                num3 = BigIntegerBuilder.AddMulCarry(ref this._rgu[num2 + i], regMul._rgu[i], uMul, num3);
            }
            if (num3 != 0u)
            {
                int num4 = num2 + regMul._iuLast + 1;
                while (num3 != 0u && num4 <= this._iuLast)
                {
                    num3 = BigIntegerBuilder.AddCarry(ref this._rgu[num4], 0u, num3);
                    num4++;
                }
                if (num3 != 0u)
                {
                    this.SetSizeKeep(this._iuLast + 2, 0);
                    this._rgu[this._iuLast] = num3;
                }
            }
        }
    }
}

...

private static uint AddMulCarry(ref uint uAdd, uint uMul1, uint uMul2, uint uCarry)
{
    ulong num = (ulong)uMul1 * (ulong)uMul2 + (ulong)uAdd + (ulong)uCarry;
    uAdd = (uint)num;
    return (uint)(num >> 32);
}