C #: Introduction to foreach and yield statements

1. foreach statement

C # The Compiler converts the foreach statement to the methods and attributes of the IEnumerable interface.

Copy codeThe Code is as follows: foreach (Person p in persons)
{
Console. WriteLine (p );
}

The foreach statement is parsed into the following code segment.

• Call the GetEnumerator () method to obtain an array enumeration.
• In the while LOOP, As long as MoveNext () returns true, the loop continues.
• Use the Current attribute to access elements in the array

Copy codeThe Code is as follows: IEnumerator enumerator = persons. GetEnumerator ();
While (enumerator. MoveNext ())
{
Person p = (Person) enumerator. Current;
Console. WriteLine (p );
}

2. yield statement

• Two forms of yield statements:

Copy codeThe Code is as follows: yield return <expression>;
Yield break;

• Use a yield return statement to return an element of the Set
• The method or attribute that contains the yield statement is an iterator. The iterator must meet the following requirements:
A. The return type must be IEnumerable, IEnumerable <T>, IEnumerator, or IEnumerator <T>.

B. It cannot have any ref or out parameters.

• The yield return statement cannot be placed in try-catch. The yield return statement can be located in try-finally try blocks.

Copy codeThe Code is as follows: try
{
// ERROR: Cannot yield a value in the boday of a try block with a catch clause
Yield return "test ";
}
Catch
{}

Try
{
//
Yield return "test again ";
}
Finally
{}

Try
{}
Finally
{
// ERROR: Cannot yield in the body of a finally clause
Yield return "";
}

The yield break statement can be located in a try block or catch block, but not in a finally block.

The following example uses the yield return statement to implement the code of a simple set and the foreach statement to iterate the set.

Copy codeThe Code is as follows: using System;
Using System. Collections. Generic;

Namespace ConsoleApplication6
{
Class Program
{
Static void Main (string [] args)
{
HelloCollection helloCollection = new HelloCollection ();
Foreach (string s in helloCollection)
{
Console. WriteLine (s );
Console. ReadLine ();
}
}
}

Public class HelloCollection
{

Public IEnumerator <String> GetEnumerator ()
{
// The yield return Statement returns an element of the set and moves it to the next element. yield break can stop iteration.
Yield return "Hello ";
Yield return "World ";
}
}
}

Use the yield return statement to implement classes that iterate sets in different ways:

Copy codeThe Code is as follows: using System;
Using System. Collections. Generic;

Namespace ConsoleApplication8
{
Class Program
{
Static void Main (string [] args)
{
MusicTitles titles = new MusicTitles ();
Foreach (string title in titles)
{
Console. WriteLine (title );
}
Console. WriteLine ();

Foreach (string title in titles. Reverse ())
{
Console. WriteLine (title );
}
Console. WriteLine ();

Foreach (string title in titles. Subset (2, 2 ))
{
Console. WriteLine (title );
Console. ReadLine ();
}
}
}

Public class MusicTitles
{
String [] names = {"a", "B", "c", "d "};
Public IEnumerator <string> GetEnumerator ()
{
For (int I = 0; I <4; I ++)
{
Yield return names [I];
}
}

Public IEnumerable <string> Reverse ()
{
For (int I = 3; I> = 0; I --)
{
Yield return names [I];
}
}

Public IEnumerable <string> Subset (int index, int length)
{
For (int I = index; I <index + length; I ++)
{
Yield return names [I];
}
}
}
}

Output: