[源代码]List的增加与删除

// Removes the element at the given index. The size of the list is
        // decreased by one.
        // 
        public void RemoveAt(int index) {
            if ((uint)index >= (uint)_size) {
                ThrowHelper.ThrowArgumentOutOfRangeException();
            }
            Contract.EndContractBlock();
            _size--;
            if (index < _size) {
                Array.Copy(_items, index + 1, _items, index, _size - index);
            }
            _items[_size] = default(T);
            _version++;
        }

可以看到 Remove的操作里涉及对List的存储空间重新赋值的工作.而多余占用的空间用Default(T)来填充并未回收

// Inserts the elements of the given collection at a given index. If
        // required, the capacity of the list is increased to twice the previous
        // capacity or the new size, whichever is larger.  Ranges may be added
        // to the end of the list by setting index to the List's size.
        //
        public void InsertRange(int index, IEnumerable<T> collection) {
            if (collection==null) {
                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
            }
            
            if ((uint)index > (uint)_size) {
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index);
            }
            Contract.EndContractBlock();
 
            ICollection<T> c = collection as ICollection<T>;
            if( c != null ) {    // if collection is ICollection<T>
                int count = c.Count;
                if (count > 0) {
                    EnsureCapacity(_size + count);
                    if (index < _size) {
                        Array.Copy(_items, index, _items, index + count, _size - index);
                    }
                    
                    // If we're inserting a List into itself, we want to be able to deal with that.
                    if (this == c) {
                        // Copy first part of _items to insert location
                        Array.Copy(_items, 0, _items, index, index);
                        // Copy last part of _items back to inserted location
                        Array.Copy(_items, index+count, _items, index*2, _size-index);
                    }
                    else {
                        T[] itemsToInsert = new T[count];
                        c.CopyTo(itemsToInsert, 0);
                        itemsToInsert.CopyTo(_items, index);                    
                    }
                    _size += count;
                }                
            }
            else {
                using(IEnumerator<T> en = collection.GetEnumerator()) {
                    while(en.MoveNext()) {
                        Insert(index++, en.Current);                                    
                    }                
                }
            }
            _version++;            
        }

 // Ensures that the capacity of this list is at least the given minimum
        // value. If the currect capacity of the list is less than min, the
        // capacity is increased to twice the current capacity or to min,
        // whichever is larger.
        private void EnsureCapacity(int min) {
            if (_items.Length < min) {
                int newCapacity = _items.Length == 0? _defaultCapacity : _items.Length * 2;
                // Allow the list to grow to maximum possible capacity (~2G elements) before encountering overflow.
                // Note that this check works even when _items.Length overflowed thanks to the (uint) cast
                if ((uint)newCapacity > Array.MaxArrayLength) newCapacity = Array.MaxArrayLength;
                if (newCapacity < min) newCapacity = min;
                Capacity = newCapacity;
            }
        }
   

// Gets and sets the capacity of this list.  The capacity is the size of
        // the internal array used to hold items.  When set, the internal 
        // array of the list is reallocated to the given capacity.
        // 
        public int Capacity {
            get {
                Contract.Ensures(Contract.Result<int>() >= 0);
                return _items.Length;
            }
            set {
                if (value < _size) {
                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.value, ExceptionResource.ArgumentOutOfRange_SmallCapacity);
                }
                Contract.EndContractBlock();
 
                if (value != _items.Length) {
                    if (value > 0) {
                        T[] newItems = new T[value];
                        if (_size > 0) {
                            Array.Copy(_items, 0, newItems, 0, _size);
                        }
                        _items = newItems;
                    }
                    else {
                        _items = _emptyArray;
                    }
                }
            }
        }

 2.1 在尾部插入速度最快

2.2 在中间插入,涉及数组拷贝

2.3 元素不够时,涉及重新分配内在和数组拷贝