C# 4 and .NET 4

142 ❘ ChaPTer 6 ArrAys And tuples
yield statement
Since the fi rst release of C#, it has been easy to iterate through collections by using the foreach statement.
With C# 1.0, it was still a lot of work to create an enumerator. C# 2.0 added the yield statement for
creating enumerators easily. The yield return statement returns one element of a collection and moves
the position to the next element, and yield break stops the iteration.
The next example shows the implementation of a simple collection using the yield return statement.
The class HelloCollection contains the method GetEnumerator() . The implementation of the
GetEnumerator() method contains two yield return statements where the strings Hello and World
are returned:
using System;
using System.Collections;
namespace Wrox.ProCSharp.Arrays
{
public class HelloCollection
{
public IEnumerator < string > GetEnumerator()
{
yield return "Hello";
yield return "World";
}
}
code snippet YieldDemo/Program.cs
A method or property that contains yield statements is also known as an iterator
block. An iterator block must be declared to return an IEnumerator or
IEnumerable interface, or the generic versions of these interfaces. This block may
contain multiple y i e l d r e t u r n or yield break statements; a return statement is
not allowed.
Now it is possible to iterate through the collection using a foreach statement:
}
public void HelloWorld()
{
var helloCollection = new HelloCollection();
foreach (var s in helloCollection)
{
Console.WriteLine(s);
}
}
With an iterator block the compiler generates a yield type, including a state machine, as shown with
the following code segment. The yield type implements the properties and methods of the interfaces
IEnumerator and IDisposable . In the sample, you can see the yield type as the inner class Enumerator .
The GetEnumerator() method of the outer class instantiates and returns a new yield type. Within the yield
type, the variable state defines the current position of the iteration and is changed every time the method
MoveNext() is invoked. MoveNext() encapsulates the code of the iterator block and sets the value of the
current variable so that the Current property returns an object depending on the position.
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