using System;
using System.Diagnostic;
using System.Collections.ObjectModel;
using System.Security.Permissions;

namespace System.Collections.Generic
{
    ...
    [Serializable()]
    public class List<t> : IList<t>, System.Collections.IList
    {
        private const int _defaultCapacity = 4;
        private T[] _items; //List<T>內(nèi)部是依靠數(shù)組_items存放數(shù)據(jù)的
        private int _size; //數(shù)組的長度
        private int _version;
        [NoSerialized]
        private Object _syncRoot;
        static T[] _emptyArray = new T[0];

        //無參數(shù)構(gòu)造函數(shù) 把_items設置成一個空的數(shù)組
        public List()
        {
            _items = _emptyArray;
        }

        //此構(gòu)造函數(shù) 給_items數(shù)組一個初始容量
        public List(int capacity)
        {
            ...
            items = new T[capaicty];
        }

        //此構(gòu)造函數(shù) 把集合類型參數(shù)拷貝給_items數(shù)組
        public List(IEnumerable<t> collection)
        {
            ...
            ICollection<t> c = collection as ICollection<t>;
            if(c != null)
            {
                int count = c.Count; //把構(gòu)造函數(shù)集合類型參數(shù)的長度賦值給臨時變量count
                _items = new T[count]; //List<T>內(nèi)部維護的_items數(shù)組的長度和構(gòu)造函數(shù)集合類型參數(shù)的長度一致
                c.CopyTo(_items, 0); //把構(gòu)造函數(shù)集合的所有元素拷貝到_items數(shù)組中去
                _size = count; //_items數(shù)組的長度就是構(gòu)造函數(shù)集合類型參數(shù)的長度
            }
            else
            {
                _size = 0;
                _items = new T[_defaultCapacity];
                ...
            }
        }

        //通過設置這個屬性，改變List<t>內(nèi)部維護的_items數(shù)組的長度
        public int Capacity
        {
            get {return _items.Length; }
            set {
                if(value != _items.Length){ //如果當前賦值和List<t>維護的內(nèi)部數(shù)組_items長度不一致
                    if(value < _size){
                        //TODO: 處理異常
                    }
                    if(value > 0){
                        T[] newItems = new T[value]; //創(chuàng)建一個臨時的、新的數(shù)組，長度為新的賦值
                        if(_size > 0){
                            //把臨時的、新的數(shù)組拷貝給List<t>內(nèi)部維護的數(shù)組_items，注意，這時_items的長度為新的賦值
                            Array.Copy(_items, 0, newItems, 0, _size); 
                        }
                    } else {
                        _items = _emptyArray;
                    }
                }
            }
        }

        public void Add(T item)
        {
            if(_size == _items.Length) EnsureCapacity(_size + 1);
            _items[_size++] = item;
            ...
        }

        //確保List<t>內(nèi)部維護的_items數(shù)組的長度至少是給定的值
        //如果_items數(shù)組原先的長度比給定的值小，就讓_items數(shù)組的長度設置為原先的長度的2倍
        privat void EnsureCapacity(int min)
        {
            if(_items.Length < min){
                 int newCapacity = _items.Length == 0 ? _defaultCapacity : _items.Legnth * 2;
                 if(newCapacity < min) newCapacity = min;
                 Capacity = newCapacity;
            }
        }

    }
}