Implementation example. This section describes a possible implementation of iterators in terms of standard C# constructs

This section describes a possible implementation of iterators in terms of standard C# constructs. The implementation described here is based on the same principles used by the Microsoft C# compiler, but it is by no means a mandated implementation or the only one possible.

The following Stack<T> class implements its GetEnumerator method using an iterator. The iterator enumerates the elements of the stack in top to bottom order.

using System;
using System.Collections;
using System.Collections.Generic;

class Stack<T>: IEnumerable<T>
{
T[] items;
int count;

public void Push(T item) {
if (items == null) {
items = new T[4];
}
else if (items.Length == count) {
T[] newItems = new T[count * 2];
Array.Copy(items, 0, newItems, 0, count);
items = newItems;
}
items[count++] = item;
}

public T Pop() {
T result = items[--count];
items[count] = default(T);
return result;
}

public IEnumerator<T> GetEnumerator() {
for (int i = count - 1; i >= 0; --i) yield return items[i];
}
}

The GetEnumerator method can be translated into an instantiation of a compiler-generated enumerator class that encapsulates the code in the iterator block, as shown in the following.

class Stack<T>: IEnumerable<T>
{
...

public IEnumerator<T> GetEnumerator() {
return new __Enumerator1(this);
}

class __Enumerator1: IEnumerator<T>, IEnumerator
{
int __state;
T __current;
Stack<T> __this;
int i;

public __Enumerator1(Stack<T> __this) {
this.__this = __this;
}

public T Current {
get { return __current; }
}

object IEnumerator.Current {
get { return __current; }
}

public bool MoveNext() {
switch (__state) {
case 1: goto __state1;
case 2: goto __state2;
}
i = __this.count - 1;
__loop:
if (i < 0) goto __state2;
__current = __this.items[i];
__state = 1;
return true;
__state1:
--i;
goto __loop;
__state2:
__state = 2;
return false;
}

public void Dispose() {
__state = 2;
}

void IEnumerator.Reset() {
throw new NotSupportedException();
}
}
}

In the preceding translation, the code in the iterator block is turned into a state machine and placed in the MoveNext method of the enumerator class. Furthermore, the local variable i is turned into a field in the enumerator object so it can continue to exist across invocations of MoveNext.

The following example prints a simple multiplication table of the integers 1 through 10. The FromTo method in the example returns an enumerable object and is implemented using an iterator.

using System;
using System.Collections.Generic;

class Test
{
static IEnumerable<int> FromTo(int from, int to) {
while (from <= to) yield return from++;
}

static void Main() {
IEnumerable<int> e = FromTo(1, 10);
foreach (int x in e) {
foreach (int y in e) {
Console.Write("{0,3} ", x * y);
}
Console.WriteLine();
}
}
}

The FromTo method can be translated into an instantiation of a compiler-generated enumerable class that encapsulates the code in the iterator block, as shown in the following.

using System;
using System.Threading;
using System.Collections;
using System.Collections.Generic;

class Test
{
...

static IEnumerable<int> FromTo(int from, int to) {
return new __Enumerable1(from, to);
}

class __Enumerable1:
IEnumerable<int>, IEnumerable,
IEnumerator<int>, IEnumerator
{
int __state;
int __current;
int __from;
int from;
int to;
int i;

public __Enumerable1(int __from, int to) {
this.__from = __from;
this.to = to;
}

public IEnumerator<int> GetEnumerator() {
__Enumerable1 result = this;
if (Interlocked.CompareExchange(ref __state, 1, 0)!= 0) {
result = new __Enumerable1(__from, to);
result.__state = 1;
}
result.from = result.__from;
return result;
}

IEnumerator IEnumerable.GetEnumerator() {
return (IEnumerator)GetEnumerator();
}

public int Current {
get { return __current; }
}

object IEnumerator.Current {
get { return __current; }
}

public bool MoveNext() {
switch (__state) {
case 1:
if (from > to) goto case 2;
__current = from++;
__state = 1;
return true;
case 2:
__state = 2;
return false;
default:
throw new InvalidOperationException();
}
}

public void Dispose() {
__state = 2;
}

void IEnumerator.Reset() {
throw new NotSupportedException();
}
}
}

The enumerable class implements both the enumerable interfaces and the enumerator interfaces, enabling it to serve as both an enumerable and an enumerator. The first time the GetEnumerator method is invoked, the enumerable object itself is returned. Subsequent invocations of the enumerable object’s GetEnumerator, if any, return a copy of the enumerable object. Thus, each returned enumerator has its own state and changes in one enumerator will not affect another. The Interlocked.CompareExchange method is used to ensure thread-safe operation.

The from and to parameters are turned into fields in the enumerable class. Because from is modified in the iterator block, an additional __from field is introduced to hold the initial value given to from in each enumerator.

The MoveNext method throws an InvalidOperationException if it is called when __state is 0. This protects against use of the enumerable object as an enumerator object without first calling GetEnumerator.

The following example shows a simple tree class. The Tree<T> class implements its GetEnumerator method using an iterator. The iterator enumerates the elements of the tree in infix order.

using System;
using System.Collections.Generic;

class Tree<T>: IEnumerable<T>
{
T value;
Tree<T> left;
Tree<T> right;

public Tree(T value, Tree<T> left, Tree<T> right) {
this.value = value;
this.left = left;
this.right = right;
}

public IEnumerator<T> GetEnumerator() {
if (left!= null) foreach (T x in left) yield x;
yield value;
if (right!= null) foreach (T x in right) yield x;
}
}

class Program
{
static Tree<T> MakeTree<T>(T[] items, int left, int right) {
if (left > right) return null;
int i = (left + right) / 2;
return new Tree<T>(items[i],
MakeTree(items, left, i - 1),
MakeTree(items, i + 1, right));
}

static Tree<T> MakeTree<T>(params T[] items) {
return MakeTree(items, 0, items.Length - 1);
}

// The output of the program is:
// 1 2 3 4 5 6 7 8 9
// Mon Tue Wed Thu Fri Sat Sun

static void Main() {
Tree<int> ints = MakeTree(1, 2, 3, 4, 5, 6, 7, 8, 9);
foreach (int i in ints) Console.Write("{0} ", i);
Console.WriteLine();

Tree<string> strings = MakeTree(
"Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun");
foreach (string s in strings) Console.Write("{0} ", s);
Console.WriteLine();
}
}

The GetEnumerator method can be translated into an instantiation of a compiler-generated enumerator class that encapsulates the code in the iterator block, as shown in the following.

class Tree<T>: IEnumerable<T>
{
...

public IEnumerator<T> GetEnumerator() {
return new __Enumerator1(this);
}

class __Enumerator1: IEnumerator<T>, IEnumerator
{
Node<T> __this;
IEnumerator<T> __left, __right;
int __state;
T __current;

public __Enumerator1(Node<T> __this) {
this.__this = __this;
}

public T Current {
get { return __current; }
}

object IEnumerator.Current {
get { return __current; }
}

public bool MoveNext() {
try {
switch (__state) {

case 0:
__state = -1;
if (__this.left == null) goto __yield_value;
__left = __this.left.GetEnumerator();
goto case 1;

case 1:
__state = -2;
if (!__left.MoveNext()) goto __left_dispose;
__current = __left.Current;
__state = 1;
return true;

__left_dispose:
__state = -1;
__left.Dispose();

__yield_value:
__current = __this.value;
__state = 2;
return true;

case 2:
__state = -1;
if (__this.right == null) goto __end;
__right = __this.right.GetEnumerator();
goto case 3;

case 3:
__state = -3;
if (!__right.MoveNext()) goto __right_dispose;
__current = __right.Current;
__state = 3;
return true;

__right_dispose:
__state = -1;
__right.Dispose();

__end:
__state = 4;
break;

}
}
finally {
if (__state < 0) Dispose();
}
return false;
}

public void Dispose() {
try {
switch (__state) {

case 1:
case -2:
__left.Dispose();
break;

case 3:
case -3:
__right.Dispose();
break;

}
}
finally {
__state = 4;
}
}

void IEnumerator.Reset() {
throw new NotSupportedException();
}
}
}

The compiler generated temporaries used in the foreach statements are lifted into the __left and __right fields of the enumerator object. The __state field of the enumerator object is carefully updated so that the correct Dispose() method will be called correctly if an exception is thrown. Note that it is not possible to write the translated code with simple foreach statements.