When to Stop Being Lazy

ToList() and ToArray() run the recipe once and keep the result. From then on you have a collection, and enumerating it a hundred times costs nothing extra.

IEnumerable<int> seq = Source();

var count = seq.Count();   // run 1
var total = seq.Sum();     // run 2

foreach (var n in seq)     // run 3
{
    Console.WriteLine(n);
}

List<int> seq = Source().ToList();

var count = seq.Count;     // no run
var total = seq.Sum();     // no run

foreach (var n in seq)     // no run
{
    Console.WriteLine(n);
}

A rule that holds up

• Returning a sequence from a method — prefer IEnumerable<T>, so the caller decides.
• Using a sequence more than once — materialise it first.
• Passing a sequence to code you do not control — materialise it, unless laziness is the point.

The awkward middle case is a method that returns IEnumerable<T> from a live resource. If the caller might enumerate twice, either document it loudly or return a List<T> and be honest about what you built.