Lazy Evaluation

　https://stackoverflow.com/questions/2515796/deferred-execution-and-eager-evaluation

　public class LazyEvaluation
    {
        public int Count { get; set; }

        public int Computation(int index)
        {
            this.Count++;

            if (index == 0)
            {
                return 0;
            }
            else if (index == 1)
            {
                return 1;
            }
            else if (index >= 2)
            {
                return Computation(index - 2) + Computation(index - 1);
            }
            else {
                throw new ArgumentException("index");
            }
        }

        public IEnumerable<int> GetComputationEager(int maxIndex)
        {
            var result = new int[maxIndex];
            for (int i = 0; i < maxIndex; i++)
            {
                result[i] = this.Computation(i);
            }
            foreach (var value in result)
            {
                yield return value;
            }
        }

        public IEnumerable<int> GetComputationImmediate(int maxIndex)
        {
            var result = new int[maxIndex];
            for (int i = 0; i < maxIndex; i++)
            {
                result[i] = this.Computation(i);
            }
            return result;
        }

        public IEnumerable<int> GetComputationLazy(int maxIndex)
        {
            for (int i = 0; i < maxIndex; i++)
            {
                yield return this.Computation(i);
            }
        }
    }