Hi-prec math for visual studio c#?

class Bignum {
	protected ulong[] limb;
	public Bignum (long initialValue) {
		limb = new ulong[] {initialValue};
	}

	public Bignum (long[] initialValue) {
		Array.Copy(limb, initialValue, initialValue.Length);
	}

	public Bignum (string initialValue) {
		// Bad method, replace with a faster one if needed
		limb = new ulong[] {long.Parse(initialValue.Substring(0, 18))};
		for (int i = 18; i < initialValue.Length; ++i)
			limb = limb * 10 + long.Parse(initialValue[i]);
	}

	public Bignum (Bignum initialValue) {
		Array.Copy(limb, initialValue.limb, initialValue.limb.Length);
	}

	public static Bignum operator+ (Bignum a, Bignum b) {
		long carry = 0;
		int sz = Math.Max (a.limb.Length. b.limb.length);

		Bignum result = new Bignum(a);
		for (int i = 0; i < sz; ++i) {
			long o1 = result.limb[i];
			long o2 = b.limb[i];
			limb[i] += b.limb[i] + carry;
			if (result.limb[i] < o1 || result.limb[i] < o2)
				carry = 1;
			else
				carry = 0;
		}
		if (carry == 1) {
			Array.Copy(result.limb, result.limb, result.limb.Length + 1);
			result.limb[sz] = 1;
		}
		return result;
	}

	public static Bignum operator* (Bignum a, Bignum b) {
		// Code here
	}

	public static Bignum operator^ (Bignum a, uint b) {
		Bignum result = new Bignum(a);
		Bignum carry = new Bignum(0);
		while (b > 1) {
			if ((b&1) == 1)
				carry *= result;
			result *= result;
			b >>= 1;
		}
		return result * carry;
	}
}