IList接口既然继承了ICollection接口，为什么还要继承IEnumerable接口..

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

using System.Collections;



namespace WindowsFormsApplication17

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();



            X x = new X();



            foreach (int i in x) // A

                MessageBox.Show(i.ToString());



            var y = from int i in x select i;

            foreach (int i in y) // B

                MessageBox.Show(i.ToString());

        }

    }



    class X : ICollection, IEnumerable

    {

        public int[] A = { 1, 2, 3 };

        public int[] B = { 4, 5, 6 };



        #region ICollection 成员



        public void CopyTo(Array array, int index)

        {

            throw new NotImplementedException();

        }



        public int Count

        {

            get { throw new NotImplementedException(); }

        }



        public bool IsSynchronized

        {

            get { throw new NotImplementedException(); }

        }



        public object SyncRoot

        {

            get { throw new NotImplementedException(); }

        }



        #endregion



        #region IEnumerable 成员



        public IEnumerator GetEnumerator()

        {

            return A.GetEnumerator();

        }



        #endregion



        #region IEnumerable 成员



        IEnumerator IEnumerable.GetEnumerator()

        {

            return B.GetEnumerator();

        }



        #endregion

    }

}

/*============================================================

**

** Interface:  IList

**

**

** Purpose: Base interface for all Lists.

**

** 

===========================================================*/

namespace System.Collections {

    

	using System;

    // An IList is an ordered collection of objects.  The exact ordering

    // is up to the implementation of the list, ranging from a sorted

    // order to insertion order.  

[System.Runtime.InteropServices.ComVisible(true)]

    public interface IList : ICollection

    {

	// Interfaces are not serializable

        // The Item property provides methods to read and edit entries in the List.

        Object this[int index] {

            get;

            set;

        }

    

        // Adds an item to the list.  The exact position in the list is 

        // implementation-dependent, so while ArrayList may always insert

        // in the last available location, a SortedList most likely would not.

        // The return value is the position the new element was inserted in.

        int Add(Object value);

    

        // Returns whether the list contains a particular item.

        bool Contains(Object value);

    

        // Removes all items from the list.

        void Clear();



	    bool IsReadOnly 

        { get; }



	

        bool IsFixedSize

        {

			get;

		}



	    

        // Returns the index of a particular item, if it is in the list.

        // Returns -1 if the item isn't in the list.

        int IndexOf(Object value);

    

        // Inserts value into the list at position index.

        // index must be non-negative and less than or equal to the 

        // number of elements in the list.  If index equals the number

        // of items in the list, then value is appended to the end.

        void Insert(int index, Object value);

    

        // Removes an item from the list.

        void Remove(Object value);

    

        // Removes the item at position index.

        void RemoveAt(int index);

    }

}