ParseHexString.cs

void Main()
{
	//	var y = new string('A', 536_870_912);
	var y = "AB4C98";
	var x = ParseHexString(y);

	x.Dump();
}

byte[] ParseHexString(string hex)
{
	var destLength = hex.Length / 2;

	var source = hex.ToCharArray();
	var dest = new byte[destLength];

	int srcIndex = 0;

	int Nibble(int c)
	{
		if (c >= 'A')
		{
			return (c - 'A') + 10;
		}
		
		return c - '0';
	}

	for (var dstIndex = 0; dstIndex < destLength; dstIndex++)
	{
		var hn = src[srcIndex++];
		if (hn == '\r' || hn == '\n')
		{
			srcIndex++;
			continue;
		}
		var b = Nibble(hn) << 4 | Nibble(source[srcIndex++]);
		dest[dstIndex] = (byte)b;
	}

	return dest;
}