C# IEnumerable接口和IEnumerator 接口-马育民老师

# 介绍

`IEnumerable` 是可以枚举的所有非泛型集合的基本接口

`IEnumerator` 支持对非泛型集合进行简单的迭代，即：迭代器

`IEnumerable` 和 `IEnumerator` 接口搭配使用，实现 **自定义集合**

# IEnumerable

命名空间: `System.Collections`

### 作用

支持 `foreach` 语句

### 类比java

相当于 JAVA `Iterable` 迭代器

# 方法

- GetEnumerator()：返回一个可用于循环访问集合的IEnumerator对象

# IEnumerator

命名空间: `System.Collections`

### 类比java

相当于 JAVA `Iterator` 接口

### 属性

- Current：获取集合中枚举器当前位置的元素。

### 方法

- MoveNext()：将枚举器推进到集合的下一个元素。

- Reset()：将枚举器设置为其初始位置，该位置位于集合中的第一个元素之前。

# 案例

### 封装类

```
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;
}
```

### 实现自定义集合

实现 `IEnumerable` 接口

```
// 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);
    }
}
```

### 实现该集合的迭代器

实现 `IEnumerator` 接口

```
// 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();
            }
        }
    }
}
```

### 测试类

```
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);
    }
}
```

相当于下面代码

People peopleList = new People(peopleArray);
        IEnumerator myie = People.GetEnumerator();
        //重置当前项，相当于把指针移到初始位置：position = -1; 一开始认识数组的索引从“0”开始
        myie.Reset();
        //向前移动一个索引，返回Bool类型，判断是否超出下标
        while (myie.MoveNext())
        {
             int i = (int)myie.Current;//从Object转成对应类型
             Console.WriteLine("Value: {0}", i);
        }
    }
}

```

# 判断 IEnumerable中没有元素

### 方式一

```
if (mylist != null && mylist.Any())
{
    // The list is not null or empty
}
```

### 方式二

```
if (mylist != null && mylist.GetEnumerator().MoveNext())
{
    // The list is not null or empty
}
```

参考：
https://www.cnblogs.com/wml-it/p/14776958.html

原文出处：http://www.malaoshi.top/show_1GW1PslTYiyt.html