for-each-ing

People mypeople = new People();
foreach (Person indivudual in mypeople)
{
   Console.WriteLine(indivudual);
}

To write a foreach loop for your collection class you need to implement IEnumerable and IEnumerator. Here’s a simple way to do it.

class Person
{
   private string _Name;

   public Person(string name)
   {
       _Name = name;
   }
   public override string ToString()
   {
       return _Name;
   }
}
class People:IEnumerable
{
   List<Person> people = new List<Person>();

   public People()
   {
       people.Add(new Person("A"));
       people.Add(new Person("B"));
       people.Add(new Person("C"));
       people.Add(new Person("D"));
   }
  
   //IEnumerable<Person> Members
   public IEnumerator GetEnumerator()
   {
       return people.GetEnumerator();
   }
}

Here instead of creating a enumerator, I am simply using the List class’s enumerator. I believe that is how you will use in most cases.

Now, you will also find IEnumerable<T> and IEnumerator<T>, they have something to do with LINQ, will post more on that later. MSDN says if you implement IEnumerable<T> then you should also implement the non-generic version of IEnumrator.

There is also something else called as iterators - if you need to return only a fragment of your collection. Heres how :

public IEnumerable<Person> ListAll()
{
   foreach (Person individual in people)
   {
       if (individual.ToString() != "B")
           yield return individual;
   }
}

This can be consumed by the earlier foreach loop and it prints out all individuals except B.

Note that we could also use yield in the GetEnumerator() method.1 question, why 2 interface to do get foreach going? because 1 interface makes sure your class can be enumerated and other interface ensure the enumeration state is maintained. If you have both together then how would 2 different loops (maybe on different threads) maintain different states.