Introduction to C#:foreach and yield statements _c# tutorials

Source: Internet
Author: User
Tags finally block readline

1. foreach statement

The C # compiler converts the foreach statement to the methods and properties of the IEnumerable interface.

Copy Code code as follows:

foreach (person p in persons)
{
Console.WriteLine (P);
}

The foreach statement resolves to the following code snippet.

• Call the GetEnumerator () method to get an enumeration of the arrays
• In the While loop, as long as MoveNext () returns True, the loop continues
• Accessing elements in an array with the current property

Copy Code code 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 Code code as follows:

Yield return <expression>;
Yield break;

• Returns an element of a collection using a yield return statement
• The method or property that contains the yield statement is an iterator. Iterators 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 located try-catch fast. The yield return statement can be located in a try block of try-finally

Copy Code code as follows:

Try
{
Error:cannot yield a value in the boday of a try blocks 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 in a try block or a catch block, but not in a finally block

The following example is the code that implements a simple collection with the yield return statement, and iterates through the collection with a foreach statement

Copy Code code 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 collection and moves to the next element; yield break can stop the iteration
Yield return "Hello";
Yield return to "world";
}
}
}

Use the yield return statement to implement a class that iterates through the collection in different ways:

Copy Code code 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:

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