While storing and retrieving list of elements we use many classes for that like ICollection,IList .

IEnumerable<T>

The read only type of collection is used to repeat the IEnumerable<T>. It gives only one method named GetEnumerator() using which user can repeat the read only elements using foreach loop. IEnumerable <T> is defined in as follows:

public void PrintAgeUpto30(IEnumerator<int> age_IEnumerator)
{
   while (age_IEnumerator.MoveNext()){
      Console.WriteLine(age_IEnumerator.Current);
      if (age_IEnumerator.Current > 20) {
         Console.WriteLine("PrintGreaterThan30 is called");
         PrintGreaterThan30(age_IEnumerator);
      }
   }
}

public void PrintGreaterThan30(IEnumerator<int> age_IEnumerator)
{
    while (age_IEnumerator.MoveNext())
        Console.WriteLine(age_IEnumerator.Current);
}
// Now Call PrintUpto30 which will call PrintGreaterThan30 
// by using our previous age IEnumerator
PrintUpto30(IEnumerator);

We use IEnumerable when we don't want to add or remove element from a collection.

We don't have any scope for changing the contents while performing any iteration over it and for iteration only for each loop  is used.