In its most basic form, an object that implements IEnumerable represents a series of objects. The objects in question can be iterated using the c# foreach keyword.

In the example below, the object sequenceOfNumbers implements IEnumerable. It represents a series of integers. The foreach loop iterates through each in turn.

int AddNumbers(IEnumerable<int> sequenceOfNumbers) {
    int returnValue = 0;
    foreach(int i in sequenceOfNumbers) {
        returnValue += i;
    }
    return returnValue;
}

Implementing the IEnumerable interface allows classes to be enumerated in the same way as BCL collections. This requires extending the Enumerator class which tracks the state of the enumeration.

Other than iterating over a standard collection, examples include:

• Using ranges of numbers based on a function rather than a collection of objects
• Implementing different iteration algorithms over collections, like DFS or BFS on a graph collection

public static void Main(string[] args) {

    foreach (var coffee in new CoffeeCollection()) {
        Console.WriteLine(coffee);
    }
}

public class CoffeeCollection : IEnumerable {
    private CoffeeEnumerator enumerator;

    public CoffeeCollection() {
        enumerator = new CoffeeEnumerator();
    }

    public IEnumerator GetEnumerator() {
        return enumerator;
    }

    public class CoffeeEnumerator : IEnumerator {
        string[] beverages = new string[3] { "espresso", "macchiato", "latte" };
        int currentIndex = -1;

        public object Current {
            get {
                return beverages[currentIndex];
            }
        }

        public bool MoveNext() {
            currentIndex++;

            if (currentIndex < beverages.Length) {
                return true;
            }

            return false;
        }

        public void Reset() {
            currentIndex = 0;
        }
    }
}