C#学习基本概念之迭代器（Iterator）

An iterator can be used to step through collections such as lists and arrays.
An iterator method or

get

accessor performs a custom iteration over a collection. An iterator method uses the yield return statement to return each element one at a time. When a

yield return

statement is reached, the current location in code is remembered.
Execution is restarted from that location the next time the iterator
function is called.
You consume an iterator from client code by using a foreach statement or by using a LINQ query.
In the following example, the first iteration of the

foreach

loop causes execution to proceed in the

SomeNumbers

iterator method until the first

yield return

statement is reached. This iteration returns a value of 3, and the
current location in the iterator method is retained. On the next
iteration of the loop, execution in the iterator method continues from
where it left off, again stopping when it reaches a

yield return

statement. This iteration returns a value of 5, and the current
location in the iterator method is again retained. The loop completes
when the end of the iterator method is reached.

static void Main()
{
foreach (int number in SomeNumbers())
{
Console.Write(number.ToString() + " ");
}
// Output: 3 5 8
Console.ReadKey();
}

public static System.Collections.IEnumerable SomeNumbers()
{
yield return 3;
yield return 5;
yield return 8;
}

The return type of an iterator method or

get

accessor can be IEnumerable, IEnumerable<T>, IEnumerator, or IEnumerator<T>.
You can use a

yield break

statement to end the iteration.
Iterators were introduced in C# in Visual Studio 2005.
In this topic
Simple Iterator

Creating a Collection Class

Using Iterators with a Generic List

Syntax Information

Technical Implementation

Use of Iterators

For all examples in this topic except the Simple Iterator example, include using directives for the

System.Collections

and

System.Collections.Generic

namespaces.

Simple Iterator
The following example has a single

yield return

statement that is inside a for loop. In

Main

, each iteration of the

foreach

statement body creates a call to the iterator function, which proceeds to the next

yield return

statement.

static void Main()
{
foreach (int number in EvenSequence(5, 18))
{
Console.Write(number.ToString() + " ");
}
// Output: 6 8 10 12 14 16 18
Console.ReadKey();
}

public static System.Collections.Generic.IEnumerable<int>
EvenSequence(int firstNumber, int lastNumber)
{
// Yield even numbers in the range.
for (int number = firstNumber; number <= lastNumber; number++)
{
if (number % 2 == 0)
{
yield return number;
}
}
}

Creating a Collection Class
In the following example, the

DaysOfTheWeek

class implements the IEnumerable interface, which requires a GetEnumerator method. The compiler implicitly calls the

GetEnumerator

method, which returns an IEnumerator.
The

GetEnumerator

method returns each string one at a time by using the

yield return

statement.

static void Main()
{
DaysOfTheWeek days = new DaysOfTheWeek();

foreach (string day in days)
{
Console.Write(day + " ");
}
// Output: Sun Mon Tue Wed Thu Fri Sat
Console.ReadKey();
}

public class DaysOfTheWeek : IEnumerable
{
private string[] days = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

public IEnumerator GetEnumerator()
{
for (int index = 0; index < days.Length; index++)
{
// Yield each day of the week.
yield return days[index];
}
}
}

The following example creates a

Zoo

class that contains a collection of animals.
The

foreach

statement that refers to the class instance (

theZoo

) implicitly calls the

GetEnumerator

method. The

foreach

statements that refer to the

Birds

and

Mammals

properties use the

AnimalsForType

named iterator method.

static void Main()
{
Zoo theZoo = new Zoo();

theZoo.AddMammal("Whale");
theZoo.AddMammal("Rhinoceros");
theZoo.AddBird("Penguin");
theZoo.AddBird("Warbler");

foreach (string name in theZoo)
{
Console.Write(name + " ");
}
Console.WriteLine();
// Output: Whale Rhinoceros Penguin Warbler

foreach (string name in theZoo.Birds)
{
Console.Write(name + " ");
}
Console.WriteLine();
// Output: Penguin Warbler

foreach (string name in theZoo.Mammals)
{
Console.Write(name + " ");
}
Console.WriteLine();
// Output: Whale Rhinoceros

Console.ReadKey();
}

public class Zoo : IEnumerable
{
// Private members.
private List<Animal> animals = new List<Animal>();

// Public methods.
public void AddMammal(string name)
{
animals.Add(new Animal { Name = name, Type = Animal.TypeEnum.Mammal });
}

public void AddBird(string name)
{
animals.Add(new Animal { Name = name, Type = Animal.TypeEnum.Bird });
}

public IEnumerator GetEnumerator()
{
foreach (Animal theAnimal in animals)
{
yield return theAnimal.Name;
}
}

// Public members.
public IEnumerable Mammals
{
get { return AnimalsForType(Animal.TypeEnum.Mammal); }
}

public IEnumerable Birds
{
get { return AnimalsForType(Animal.TypeEnum.Bird); }
}

// Private methods.
private IEnumerable AnimalsForType(Animal.TypeEnum type)
{
foreach (Animal theAnimal in animals)
{
if (theAnimal.Type == type)
{
yield return theAnimal.Name;
}
}
}

// Private class.
private class Animal
{
public enum TypeEnum { Bird, Mammal }

public string Name { get; set; }
public TypeEnum Type { get; set; }
}
}

Using Iterators with a Generic List
In the following example, the

Stack(Of T)

generic class implements the IEnumerable<T> generic interface. The

Push

method assigns values to an array of type

T

. The GetEnumerator method returns the array values by using the

yield return

statement.
In addition to the generic GetEnumerator method, the non-generic GetEnumerator method must also be implemented. This is because IEnumerable<T> inherits from IEnumerable. The non-generic implementation defers to the generic implementation.
The
example uses named iterators to support various ways of iterating
through the same collection of data. These named iterators are the

TopToBottom

and

BottomToTop

properties, and the

TopN

method.
The

BottomToTop

property uses an iterator in a

get

accessor.

static void Main()
{
Stack<int> theStack = new Stack<int>();

//  Add items to the stack.
for (int number = 0; number <= 9; number++)
{
theStack.Push(number);
}

// Retrieve items from the stack.
// foreach is allowed because theStack implements
// IEnumerable<int>.
foreach (int number in theStack)
{
Console.Write("{0} ", number);
}
Console.WriteLine();
// Output: 9 8 7 6 5 4 3 2 1 0

// foreach is allowed, because theStack.TopToBottom
// returns IEnumerable(Of Integer).
foreach (int number in theStack.TopToBottom)
{
Console.Write("{0} ", number);
}
Console.WriteLine();
// Output: 9 8 7 6 5 4 3 2 1 0

foreach (int number in theStack.BottomToTop)
{
Console.Write("{0} ", number);
}
Console.WriteLine();
// Output: 0 1 2 3 4 5 6 7 8 9

foreach (int number in theStack.TopN(7))
{
Console.Write("{0} ", number);
}
Console.WriteLine();
// Output: 9 8 7 6 5 4 3

Console.ReadKey();
}

public class Stack<T> : IEnumerable<T>
{
private T[] values = new T[100];
private int top = 0;

public void Push(T t)
{
values[top] = t;
top++;
}
public T Pop()
{
top--;
return values[top];
}

// This method implements the GetEnumerator method. It allows
// an instance of the class to be used in a foreach statement.
public IEnumerator<T> GetEnumerator()
{
for (int index = top - 1; index >= 0; index--)
{
yield return values[index];
}
}

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

public IEnumerable<T> TopToBottom
{
get { return this; }
}

public IEnumerable<T> BottomToTop
{
get
{
for (int index = 0; index <= top - 1; index++)
{
yield return values[index];
}
}
}

public IEnumerable<T> TopN(int itemsFromTop)
{
// Return less than itemsFromTop if necessary.
int startIndex = itemsFromTop >= top ? 0 : top - itemsFromTop;

for (int index = top - 1; index >= startIndex; index--)
{
yield return values[index];
}
}

}

Syntax Information
An iterator can occur as a method or

get

accessor. An iterator cannot occur in an event, instance constructor, static constructor, or static destructor.
An implicit conversion must exist from the expression type in the

yield return

statement to the return type of the iterator.
In C#, an iterator method cannot have any

ref

or

out

parameters.
In C#, "yield" is not a reserved word and has special meaning only when it is used before a

return

or

break

keyword.

Technical Implementation
Although
you write an iterator as a method, the compiler translates it into a
nested class that is, in effect, a state machine. This class keeps track
of the position of the iterator as long the

foreach

loop in the client code continues.
To
see what the compiler does, you can use the Ildasm.exe tool to view the
Microsoft intermediate language code that is generated for an iterator
method.
When you create an iterator for a class or struct, you do not have to implement the whole IEnumerator interface. When the compiler detects the iterator, it automatically generates the

Current

,

MoveNext

, and

Dispose

methods of the IEnumerator or IEnumerator<T> interface.
On each successive iteration of the

foreach

loop (or the direct call to

IEnumerator.MoveNext

), the next iterator code body resumes after the previous

yield return

statement. It then continues to the next

yield return

statement until the end of the iterator body is reached, or until a

yield break

statement is encountered.
Iterators do not support the IEnumerator.Reset method. To re-iterate from the start, you must obtain a new iterator.
For additional information, see the C# Language Specification.

Use of Iterators
Iterators enable you to maintain the simplicity of a

foreach

loop when you need to use complex code to populate a list sequence. This can be useful when you want to do the following:
Modify the list sequence after the first

foreach

loop iteration.

Avoid fully loading a large list before the first iteration of a

foreach

loop. An example is a paged fetch to load a batch of table rows. Another example is the EnumerateFiles method, which implements iterators within the .NET Framework.

Encapsulate
building the list in the iterator. In the iterator method, you can
build the list and then yield each result in a loop.

备注：转载自：https://msdn.microsoft.com/zh-cn/library/mt639331.aspx