关于IEnumerable，大家帮我看看

[ninglh]

下面是下载的关于IEnumerable的实例，我看不懂，哪位大哥帮忙解释下，讲讲如何实现IEnumerable接口，谢了
class Employees : IEnumerable
{
private ArrayList m_Employees;
private int m_MaxEmployees;

public Employees( int MaxEmployees )
{
m_MaxEmployees = MaxEmployees;
m_Employees = new ArrayList( MaxEmployees );
}

// Here is the implementation of the indexer by array index
public Employee this[int index]
{
get
{
// Check for out of bounds condition
if ( index < 0 || index > m_Employees.Count - 1 )
return null;

// Return employee based on index passed in
return (Employee) m_Employees[index];
}

set
{
// Check for out of bounds condition
if ( index < 0 || index > m_MaxEmployees-1 )
return;

// Add new employee
m_Employees.Insert( index, value );
}
}

// Here is the implementation of the indexer by SSN
public Employee this[string SSN]
{
get
{
Employee empReturned = null;

foreach ( Employee employee in m_Employees )
{
// Return employee based on index passed in
if ( employee.SSN == SSN )
{
empReturned = employee;
break;
}
}

return empReturned;
}
}

// Return the total number of employees.
public int Length
{
get
{
return m_Employees.Count;
}
}

// IEnumerable implementation, delegates IEnumerator to
// the ArrayList
public IEnumerator GetEnumerator()
{
return m_Employees.GetEnumerator();
}
}

class Employee
{
private string m_firstName;
private string m_middleName;
private string m_lastName;
private string m_SSN;

// Constructor
public Employee( string FirstName, string LastName, string
MiddleName, string SSN )
{
m_firstName = FirstName;
m_middleName = MiddleName;
m_lastName = LastName;
m_SSN = SSN;
}

// FirstName property
public string FirstName
{
get { return m_firstName; }
set { m_firstName = value; }
}

// MiddleName property
public string MiddleName
{
get { return m_middleName; }
set { m_middleName = value; }
}

// LastName property
public string LastName
{
get { return m_lastName; }
set { m_lastName = value; }
}

// SSN property
public string SSN
{
get { return m_SSN; }
set { m_SSN = value; }
}
}

[ninglh]

谢谢上面两位大哥的发言,能不能再说清楚点,主要关于GetEnumerator方法的,前面的我已经知道了,这个例子前面是把讲索引部分加在里面了,IEnumerable的运用只是用到了里面的
// IEnumerable implementation, delegates IEnumerator to
// the ArrayList
public IEnumerator GetEnumerator()
{
return m_Employees.GetEnumerator();
}

[vwxyzh]

可以参考一下ArrayList的实现：

public virtual IEnumerator GetEnumerator()
{
    return new ArrayListEnumeratorSimple(this);
}

其中ArrayListEnumeratorSimple类的实现（有点复杂，不需要全看懂）：

[Serializable]
private sealed class ArrayListEnumeratorSimple : IEnumerator, ICloneable
{
    private object currentElement;
    private static object dummyObject = new object();
    private int index;
    [NonSerialized]
    private bool isArrayList;
    private ArrayList list;
    private int version;

    internal ArrayListEnumeratorSimple(ArrayList list)
    {
        this.list = list;
        this.index = -1;
        this.version = list._version;
        this.isArrayList = list.GetType() == typeof(ArrayList);
        this.currentElement = dummyObject;
    }

    public object Clone()
    {
        return base.MemberwiseClone();
    }

    public bool MoveNext()
    {
        if (this.version != this.list._version)
        {
            throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumFailedVersion"));
        }
        if (this.isArrayList)
        {
            if (this.index < (this.list._size - 1))
            {
                this.currentElement = this.list._items[++this.index];
                return true;
            }
            this.currentElement = dummyObject;
            this.index = this.list._size;
            return false;
        }
        if (this.index < (this.list.Count - 1))
        {
            this.currentElement = this.list[++this.index];
            return true;
        }
        this.index = this.list.Count;
        this.currentElement = dummyObject;
        return false;
    }

    public void Reset()
    {
        if (this.version != this.list._version)
        {
            throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumFailedVersion"));
        }
        this.currentElement = dummyObject;
        this.index = -1;
    }

    public object Current
    {
        get
        {
            object currentElement = this.currentElement;
            if (dummyObject != currentElement)
            {
                return currentElement;
            }
            if (this.index == -1)
            {
                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumNotStarted"));
            }
            throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumEnded"));
        }
    }
}

其中核心成员是MoveNext()和Current
MoveNext有点复杂（因为额外做了版本检查等事情），简单的说就是记录一个index，每次MoveNext()就把index++，取出ArrayList第index处的元素，放在currentElement里面，返回true，如果无法MoveNext（就是后面没有元素了），就返回false
再取Current属性时返回currentElement

[virusswb]

不是像数组一样，那是this索引器的功能

这个接口是你可以在你的类实例上使用foreach来枚举

[vwxyzh]

不一样，例如Stack就只能foreach遍历，不能用索引器

[brucenan999]

你可以像数组一样操作Employees

Employees[i]
Employees.Length

继承IEnumerable 就是提供这样的功能.

[vwxyzh]

错了，这个例子不是很好
IEnumerable是为foreach量身定做的
核心方法就是GetEnumerator，用来获得一个IEnumarator的实例。
（例子中没有这个部分的实现，所以说不好。）

至于索引器、Length这些都和IEnumerable无关

[ninglh]

怎么结贴啊