关于IEnumerable，大家帮我看看
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; }
  }
}

------解决方案--------------------
你可以像数组一样操作Employees 

Employees[i]