- List
// C# 源码
public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>
{
private const int _defaultCapacity = 4;//默认容量为4
private T[] _items;//list内部是用数组实现的
[ContractPublicPropertyName("Count")]
private int _size;
private int _version;
[NonSerialized]
private Object _syncRoot;
static readonly T[] _emptyArray = new T[0];
// 其他内容
}
// C# Code
// Adds the given object to the end of this list. The size of the list is
// increased by one. If required, the capacity of the list is doubled
// before adding the new element.
//
//扩容时翻倍
public void Add(T item) {
if (_size == _items.Length)
EnsureCapacity(_size + 1);
_items[_size++] = item;
_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;
}
}
}
}
C# 中 List默认的容量其实是4，所以最好还是初始化容量吧，可以想象，如果一个列表里面有129个元素，那么代码中对Capacity的调用会有很多次，4->8->16->32->64->128->256，不但最后的容量中产生了大量的浪费，前面的一堆对象也都需要GC搞定了。也就是252个对象。浪费还是很严重的。
// Removes the element at the given index. The size of the list is
// decreased by one.
public bool Remove(T item) {
int index = IndexOf(item);
if (index >= 0) {
RemoveAt(index);
return true;
}
return false;
}
// Returns the index of the first occurrence of a given value in a range of
// this list. The list is searched forwards from beginning to end.
// The elements of the list are compared to the given value using the
// Object.Equals method.
//
// This method uses the Array.IndexOf method to perform the
// search.
//
public int IndexOf(T item) {
Contract.Ensures(Contract.Result<int>() >= -1);
Contract.Ensures(Contract.Result<int>() < Count);
return Array.IndexOf(_items, item, 0, _size);
}
// Array.cs
public static int IndexOf<T>(T[] array, T value, int startIndex, int count) {
if (array==null) {
throw new ArgumentNullException("array");
}
if (startIndex < 0 || startIndex > array.Length ) {
throw new ArgumentOutOfRangeException("startIndex",
Environment.GetResourceString("ArgumentOutOfRange_Index"));
}
if (count < 0 || count > array.Length - startIndex) {
throw new ArgumentOutOfRangeException("count",
Environment.GetResourceString("ArgumentOutOfRange_Count"));
}
Contract.Ensures(Contract.Result<int>() < array.Length);
Contract.EndContractBlock();
return EqualityComparer<T>.Default.IndexOf(array, value, startIndex, count);
}
// 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++;
}
从代码来看，remove操作优先看的是能否找到该元素，如果能找到，将其移除，返回True，否则，返回false
C#的索引方法有点复杂，点到EqualityComparer里面看了一下索引的方法，这也是C#跟Java的不同之处了，Java的泛型里面是不能写入Primitive类型的，因为Primitive类型其实是不继承Object的，所以无法调用其中的equals方法。
但是C#是支持的，所以，会判断元类型的Type，然后选取对应的Equals方法。
现在回头看下RemoveAt方法，该方法仍然会调用Array.Copy操作，所以，可想而知删除操作的复杂度了，内存中平均删除一个元素，要移动n/2个元素，复杂度为O(n)
而RemoveAll方法本身是复杂度为O(n)的，所以最好不要在循环中写Remove操作吧。