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PART II
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Implementing IEnumerable and IEnumerator
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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 this interface 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:
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// Implement IEnumerable and IEnumerator using System; using SystemCollections; class MyClass : IEnumerator, IEnumerable { char[] chrs = { 'A', 'B', 'C', 'D' }; int idx = -1; // Implement IEnumerable
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Part II:
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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 == chrsLength-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) ConsoleWrite(ch + " "); ConsoleWriteLine(); // Display the contents of mc, again foreach(char ch in mc) ConsoleWrite(ch + " "); ConsoleWriteLine(); } }
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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( )
25:
Collections, Enumerators, and Iterators
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
PART II
Using Iterators
As the preceding example shows, it is not difficult to implement IEnumerator and IEnumerable However, it can be made even easier through the use of an iterator An iterator is a method, operator, or accessor that returns the members of a set of objects, one member at a time, from start to finish For example, assuming some array that has five elements, then an iterator for that array will return those five elements, one at a time Implementing an iterator is another way to make it possible for an object of a class to be used in a foreach loop Let s begin with an example of a simple iterator The following program is a modified version of the preceding program that uses an iterator rather than explicitly implementing IEnumerator and IEnumerable
// A simple example of an iterator using System; using SystemCollections; class MyClass { char[] chrs = { 'A', 'B', 'C', 'D' }; // This iterator returns the characters // in the chrs array public IEnumerator GetEnumerator() { foreach(char ch in chrs) yield return ch; } } class ItrDemo { static void Main() { MyClass mc = new MyClass(); foreach(char ch in mc) ConsoleWrite(ch + " "); ConsoleWriteLine(); } }
Part II:
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