alig.cs

void Main()
{
	var a = new BigDecimal(
		BigInteger.Parse("9999999999999999999999999999999999999999999999999"), 0);
		
		
	var b = new BigDecimal(
		BigInteger.Parse("99999999999999999999998999999988998899999999999"),
		17);
		
	var c = a*b;
	// = 9999999999999999999999899999998899889999999999899000000000000000000000010000000.11001100000000001
	Console.WriteLine(c); 
}

public struct BigDecimal
{
	public BigInteger Integer { get; set; }
	public BigInteger Scale { get; set; }

	public BigDecimal(BigInteger integer, BigInteger scale) : this()
	{
		Integer = integer;
		Scale = scale;
		while (Scale > 0 && Integer % 10 == 0)
		{
			Integer /= 10;
			Scale -= 1;
		}
	}

	public static implicit operator BigDecimal(decimal a)
	{
		BigInteger integer = (BigInteger)a;
		BigInteger scale = 0;
		decimal scaleFactor = 1m;
		while ((decimal)integer != a * scaleFactor)
		{
			scale += 1;
			scaleFactor *= 10;
			integer = (BigInteger)(a * scaleFactor);
		}
		return new BigDecimal(integer, scale);
	}

	public static BigDecimal operator *(BigDecimal a, BigDecimal b)
	{
		return new BigDecimal(a.Integer * b.Integer, a.Scale + b.Scale);
	}

	public override string ToString()
	{
		string s = Integer.ToString();
		if (Scale != 0)
		{
			if (Scale > Int32.MaxValue) return "[Undisplayable]";
			int decimalPos = s.Length - (int)Scale;
			s = s.Insert(decimalPos, decimalPos == 0 ? "0." : ".");
		}
		return s;
	}
}