Work You Never Do

Chain iterators together and the laziness composes. Nothing in the chain does work until the end of the chain is pulled — and it pulls only as much as it needs.

var firstBig = Naturals()
    .Where(n => n % 7 == 0)
    .Select(n => n * n)
    .First(n => n > 1000);

Console.WriteLine(firstBig);   // 35 * 35
   

1225

Where and Select are iterators too — that is why LINQ behaves this way. The whole chain produced exactly as many values as it took to satisfy First, and then stopped mid-stream.

An eager version would have built three lists, two of them infinite.

var all = new List<int>();
for (int i = 1; i <= 1000000; i++)
    all.Add(i);

var sevens = new List<int>();
foreach (var n in all)
    if (n % 7 == 0) sevens.Add(n);

var squares = new List<int>();
foreach (var n in sevens)
    squares.Add(n * n);

var answer = squares.First(n => n > 1000);

var answer = Naturals()
    .Where(n => n % 7 == 0)
    .Select(n => n * n)
    .First(n => n > 1000);