C# 0 The Complete Reference

IEnumerator

 and 

IEnumerable

, using either their generic or non-generic form. Fortunately, because these 

interfaces are so small, they are easy to implement.

N

OTE

N

OTE

Technically, in order for a class to be used with a 

foreach

 loop, it does not actually have to 

specify

IEnumerator

or

IEnumerable

 as implemented interfaces. It does, however, have to 

provide their methods, which are 

GetEnumerator( )

,

Reset( )

,

MoveNext( )

, and the property 

Current

. However, not specifying these interfaces reduces the usability of the class in a mixed-

language environment.

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 {

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      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 {

  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.

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