TestClass1.cs

using System;

public class Class1
{
	static void Main(string[] args)
	{
		int i, j, n;
		ulong x;
		var f = new ulong[16];
		var bigs = new ulong[] { 16758243290880UL, 24959017348650UL, 14593825548650UL, 0 };
		ulong[] tens = new ulong[20];
		tens[0] = 1;
		for (i = 1; i < 20; i++)
			tens[i] = tens[i - 1] * 10;

		for (x = 1, n = 0; n < 25; x++)
		{
			if ((j = fangs(x, f, tens)) == 0) continue;
			Console.Write(++n + ": ");
			show_fangs(x, f, j);
		}

		Console.WriteLine();
		for (i = 0; bigs[i] > 0; i++)
		{
			if ((j = fangs(bigs[i], f, tens)) > 0)
				show_fangs(bigs[i], f, j);
			else
				Console.WriteLine(bigs[i] + " is not vampiric.");
		}
		Console.ReadLine();
	}

	private static void show_fangs(ulong x, ulong[] f, int cnt)
	{
		Console.Write(x);
		int i;
		for (i = 0; i < cnt; i++)
			Console.Write(" = " + f[i] + " * " + (x / f[i]));
		Console.WriteLine();
	}

	private static int fangs(ulong x, ulong[] f, ulong[] tens)
	{
		int n = 0;
		int nd = ndigits(x);
		if ((nd & 1) > 0) return 0;
		nd /= 2;

		ulong lo, hi;
		lo = Math.Max(tens[nd - 1], (x + tens[nd] - 2) / (tens[nd] - 1));
		hi = Math.Min(x / lo, (ulong)Math.Sqrt(x));

		ulong a, b, t = dtally(x);
		for (a = lo; a <= hi; a++)
		{
			b = x / a;
			if (a * b == x && ((a % 10) > 0 || (b % 10) > 0) && t == dtally(a) + dtally(b))
				f[n++] = a;
		}

		return n;
	}

	private static ulong dtally(ulong x)
	{
		ulong t = 0;
		while (x > 0)
		{
			t += 1UL << (int)((x % 10) * 6);
			x /= 10;
		}

		return t;
	}

	private static int ndigits(ulong x)
	{
		int n = 0;
		while (x > 0)
		{
			n++;
			x /= 10;
		}
		return n;
	}
}