What is Duck typing and when can we foreach iterate through our own collection?

Question

C++ / C#

When can we iterate through our own collection with foreach?
What needs to be done for this and why? (Tell about duck typing)

Answer 1

As I understand it, the question is about the fact that it is absolutely not necessary to implement IEnumerable to pass through the collection through foreach - it is enough to write a couple of methods:

public class MyEnumerable {
  public MyEnumerator GetEnumerator() => new MyEnumerator();
}
public class MyEnumerator {
  // Этот метод должен переходить к следующему элементу, и возвращать true, если переход произошёл,
  // и false, если дальше элементов нет
  public bool MoveNext() => false;

  // Это свойство должно возвращать текущий элемент
  public int Current => 0;
}

Then it will be possible to shove this into the forich:

foreach(int x in new MyEnumerable()) {
  Console.WriteLine(x); // вообще-то мы сюда не попадём, тк Next всегда возвращает false, но синтаксической ошибки нет.
}

Initially, this was invented in order not to cause unnecessary element boxing when iterating over arrays, because initially there were no generics in C# => there was no generic IEnumerable<T>
But now it makes sense to use it only when you want to iterate by ref struct because they cannot implement interfaces.