C# Feature – Iterator (yield)

Overview

Iterators let you write lazy sequences using yield return and yield break.

Introduced In

C# 2 (2005)

Before

public IEnumerable<int> Evens(int max)
{
  var list = new List<int>();
  for (int i=0; i<=max; i++) if (i%2==0) list.Add(i);
  return list;
}

After

public IEnumerable<int> Evens(int max)
{
  for (int i=0; i<=max; i++)
    if (i%2==0) yield return i;
}

Gotchas & Best Practices

• Deferred execution; exceptions surface on enumeration.
• Can’t mix yield with ref/out parameters or unsafe blocks in the same method.