А вам слабо найти тест, на котором эта штука не работает? :-)

Heap.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace TestingTask
{
	public class Heap<T>
	{
		private readonly List<T> data = new List<T>(10);
		private readonly Func<T, int> score;

		public Heap(Func<T, int> score)
		{
			this.score = score;
		}

		public int Count
		{
			get { return data.Count; }
		}

		public T PeekBest()
		{
			if (Count == 0) throw new InvalidOperationException("Heap is empty");
			return data[0];
		}

		public T ExtractBest()
		{
			if (Count == 0) throw new InvalidOperationException("Heap is empty");
			T res = data[0];
			data[0] = data[data.Count - 1];
			data.RemoveAt(data.Count - 1);
			Down(0);
			return res;
		}

		public void Add(T item)
		{
			data.Add(item);
			Up(data.Count - 1);
		}

		private void Up(int i)
		{
			if (i == 0) return;
			int parent = i/2;
			if (score(data[parent]) < score(data[i]))
			{
				Swap(parent, i);
				Up(parent);
			}
		}

		private void Swap(int j, int i)
		{
			T t = data[j];
			data[j] = data[i];
			data[i] = t;
		}

		private void Down(int i)
		{
			int c1 = i*2;
			int c2 = i*2 + 1;
			if (c1 >= data.Count) return; // уже на дне!
			int c = (c2 < data.Count && score(data[c2]) > score(data[c1])) ? c2 : c1;
			if (score(data[i]) < score(data[c]))
			{
				Swap(i, c);
				Down(c);
			}
		}
	}
}