Implementing IEnumerable and IEnumerator

As mentioned earlier, normally it is easier (and better) to use a foreach loop to cycle through a collection than it is to explicitly use IEnumerator methods. However, understanding the operation of these interfaces is important for another reason: If you want to create a class that contains objects that can be enumerated via a foreach loop, then that class must implement IEnumerator. It must also implement IEnumerable. In other words, to enable an object of a class that you create to be used in a foreach loop, you must implement IEnumerator and IEnumerable, using either their generic or non-generic form. Fortunately, because these interfaces are so small, they are easy to implement.

Here is an example that implements the non-generic versions of IEnumerable and IEnumerator so that the contents of the array encapsulated within MyClass can be enumerated.

 // Implement IEnumerable and IEnumerator. using System; using System.Collections; class MyClass : IEnumerator, IEnumerable {   char[] chrs = { 'A', 'B', 'C', 'D' };   int idx = -1;   // Implement IEnumerable.   public IEnumerator GetEnumerator() {     return this;   }   // The following methods implement IEnumerator.   // Return the current object.   public object Current {     get {       return chrs[idx];     }   }   // Advance to the next object.   public bool MoveNext() {     if(idx == chrs.Length-1) {       Reset(); // reset enumerator at the end.       return false;     }     idx++;     return true;   }   // Reset the enumerator to the start.   public void Reset() { idx = -1; } } class EnumeratorImplDemo {   public static void Main() {     MyClass mc = new MyClass();     // Display the contents of mc.     foreach(char ch in mc)       Console.Write(ch + " ");     Console.WriteLine();     // Display the contents of mc, again.     foreach(char ch in mc)       Console.Write(ch + " ");     Console.WriteLine();   } }

Here is the output:

 A B C D A B C D

In the program, first examine MyClass. It encapsulates a small char array that contains the characters A through D. An index into this array is stored in idx, which is initialized to −1. MyClass then implements both IEnumerator and IEnumerable. GetEnumerator( ) returns a reference to the enumerator, which in this case is the current object. The Current property returns the next character in the array, which is the object at idx. The MoveNext( ) method advances idx to the next location. It returns false if the end of the collection has been reached and true otherwise. Reset( ) sets idx to −1. Recall that an enumerator is undefined until after the first call to MoveNext( ). Thus, in a foreach loop, MoveNext( ) is automatically called before Current. This is why idx must initially be −1; it is advanced to zero when the foreach loop begins. A generic implementation would work in a similar fashion.

Inside Main( ), an object of type MyClass called mc is created, and the contents of the object are twice displayed by use of a foreach loop.