Detailed explanation of the principle of foreach implementation in C#

This article mainly introduces the foreach implementation principle in C# in detail, which has certain reference value. Interested friends can refer to it

This article mainly records my learning of the foreach traversal principle in C# experience.

Traversing elements in a collection is an operation often involved in all coding, so most programming languages ​​have written this process into the grammar, such as foreach in C#. It is common to see the following traversal code:

var lstStr = new List<string> { "a", "b" };
   foreach (var str in lstStr)
      {
        Console.WriteLine(str);
      }

The actual execution process of this code:

var lstStr = new List<string> {"a", "b"};
   IEnumerator<string> enumeratorLst = lstStr.GetEnumerator();
   while (enumeratorLst.MoveNext())
      {
        Console.WriteLine(enumeratorLst.Current);
      }

will find that there is the GetEnumerator() method and the IEnumerator type, which involves the concepts of enumerable types and enumerators.

In order to facilitate understanding, the following is a non-generic example:

// 摘要:
//   公开枚举器，该枚举器支持在非泛型集合上进行简单迭代。
  public interface IEnumerable
  {
    // 摘要:
    //   返回一个循环访问集合的枚举器。
    //
    // 返回结果:
    //   可用于循环访问集合的 System.Collections.IEnumerator 对象。
    IEnumerator GetEnumerator();
  }

The class that implements this interface is called an enumerable type, which is A flag that can be traversed using foreach.

The return value of the method GetEnumerator() is an enumerator, which can be understood as a cursor.

// 摘要:
//   支持对非泛型集合的简单迭代。
  public interface IEnumerator
  {
    // 摘要:
    //   获取集合中的当前元素。
    //
    // 返回结果:
    //   集合中的当前元素。
    //
    // 异常:
    //  System.InvalidOperationException:
    //   枚举数定位在该集合的第一个元素之前或最后一个元素之后。
    object Current { get; }

    // 摘要:
    //   将枚举数推进到集合的下一个元素。
    //
    // 返回结果:
    //   如果枚举数成功地推进到下一个元素，则为 true；如果枚举数越过集合的结尾，则为 false。
    //
    // 异常:
    //  System.InvalidOperationException:
    //   在创建了枚举数后集合被修改了。
    bool MoveNext();
    //
    // 摘要:
    //   将枚举数设置为其初始位置，该位置位于集合中第一个元素之前。
    //
    // 异常:
    //  System.InvalidOperationException:
    //   在创建了枚举数后集合被修改了。
    void Reset();
  }

The following is an example of customizing an iterator (https://msdn.microsoft.com/en-us/library/system.collections.ienumerator .aspx):

using System;
using System.Collections;

// Simple business object.
public class Person
{
  public Person(string fName, string lName)
  {
    this.firstName = fName;
    this.lastName = lName;
  }

  public string firstName;
  public string lastName;
}

// Collection of Person objects. This class
// implements IEnumerable so that it can be used
// with ForEach syntax.
public class People : IEnumerable
{
  private Person[] _people;
  public People(Person[] pArray)
  {
    _people = new Person[pArray.Length];

    for (int i = 0; i < pArray.Length; i++)
    {
      _people[i] = pArray[i];
    }
  }

// Implementation for the GetEnumerator method.
  IEnumerator IEnumerable.GetEnumerator()
  {
    return (IEnumerator) GetEnumerator();
  }

  public PeopleEnum GetEnumerator()
  {
    return new PeopleEnum(_people);
  }
}

// When you implement IEnumerable, you must also implement IEnumerator.
public class PeopleEnum : IEnumerator
{
  public Person[] _people;

  // Enumerators are positioned before the first element
  // until the first MoveNext() call.
  int position = -1;

  public PeopleEnum(Person[] list)
  {
    _people = list;
  }

  public bool MoveNext()
  {
    position++;
    return (position < _people.Length);
  }

  public void Reset()
  {
    position = -1;
  }

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

  public Person Current
  {
    get
    {
      try
      {
        return _people[position];
      }
      catch (IndexOutOfRangeException)
      {
        throw new InvalidOperationException();
      }
    }
  }
}

class App
{
  static void Main()
  {
    Person[] peopleArray = new Person[3]
    {
      new Person("John", "Smith"),
      new Person("Jim", "Johnson"),
      new Person("Sue", "Rabon"),
    };

    People peopleList = new People(peopleArray);
    foreach (Person p in peopleList)
      Console.WriteLine(p.firstName + " " + p.lastName);

  }
}

/* This code produces output similar to the following:
 *
 * John Smith
 * Jim Johnson
 * Sue Rabon
 *
 */

After having the yield keyword, we can create an enumerator in this way:

using System;
using System.Collections;

// Simple business object.
public class Person
{
  public Person(string fName, string lName)
  {
    this.firstName = fName;
    this.lastName = lName;
  }

  public string firstName;
  public string lastName;
}

// Collection of Person objects. This class
// implements IEnumerable so that it can be used
// with ForEach syntax.
public class People : IEnumerable
{
  private Person[] _people;

  public People(Person[] pArray)
  {
    _people = new Person[pArray.Length];

    for (int i = 0; i < pArray.Length; i++)
    {
      _people[i] = pArray[i];
    }
  }

  // Implementation for the GetEnumerator method.
  IEnumerator IEnumerable.GetEnumerator()
  {
    for (int i = 0; i < _people.Length; i++)
    {
      yield return _people[i];
    }
  }

}


class App
{
  static void Main()
  {
    Person[] peopleArray = new Person[3]
    {
      new Person("John", "Smith"),
      new Person("Jim", "Johnson"),
      new Person("Sue", "Rabon"),
    };

    People peopleList = new People(peopleArray);
    foreach (Person p in peopleList)
      Console.WriteLine(p.firstName + " " + p.lastName);
  }
}

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