List

// C# 源碼

public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>

{

??? private const int _defaultCapacity = 4;//默認(rèn)容量為4

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??? private T[] _items;//list內(nèi)部是用數(shù)組實(shí)現(xiàn)的

??? [ContractPublicPropertyName("Count")]

??? private int _size;

??? private int _version;//在遍歷時如果發(fā)現(xiàn)_version變了立即退出并拋出遍歷過程集合被修改異常，比如在foreach里remove或add元素就會導(dǎo)致這個異常。更常見的是出現(xiàn)在多線程時一個線程遍歷集合，另一個線程修改集合的時候，相信很多人吃過苦頭。

??? [NonSerialized]

??? private Object _syncRoot;

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??? static readonly T[]? _emptyArray = new T[0];???????

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??? // 其他內(nèi)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.

//

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//擴(kuò)容時翻倍

public void Add(T item) {

??? if (_size == _items.Length)

??????? EnsureCapacity(_size + 1);

??? _items[_size++] = item;

??? _version++;

}

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// 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;

??? }

}

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// 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默認(rèn)的容量其實(shí)是4，所以最好還是初始化容量吧，可以想象，如果一個列表里面有129個元素，那么代碼中對Capacity的調(diào)用會有很多次，4->8->16->32->64->128->256，不但最后的容量中產(chǎn)生了大量的浪費(fèi)，前面的一堆對象也都需要GC搞定了。也就是252個對象。浪費(fèi)還是很嚴(yán)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;

??? }

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??? return false;

}

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// 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);

}

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// Array.cs

public static int IndexOf<T>(T[] array, T value, int startIndex, int count) {

??? if (array==null) {

??????? throw new ArgumentNullException("array");

??? }

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??? if (startIndex < 0 || startIndex > array.Length ) {

??????? throw new ArgumentOutOfRangeException("startIndex",

??????????? Environment.GetResourceString("ArgumentOutOfRange_Index"));

??? }

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??? if (count < 0 || count > array.Length - startIndex) {

?? ?????throw new ArgumentOutOfRangeException("count",

??????????? Environment.GetResourceString("ArgumentOutOfRange_Count"));

??? }

??? Contract.Ensures(Contract.Result<int>() < array.Length);

??? Contract.EndContractBlock();

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??? return EqualityComparer<T>.Default.IndexOf(array, value, startIndex, count);

}

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// 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++;

}

從代碼來看，remove操作優(yōu)先看的是能否找到該元素，如果能找到，將其移除，返回True，否則，返回false

C#的索引方法有點(diǎn)復(fù)雜，點(diǎn)到EqualityComparer里面看了一下索引的方法，這也是C#跟Java的不同之處了，Java的泛型里面是不能寫入Primitive類型的，因?yàn)镻rimitive類型其實(shí)是不繼承Object的，所以無法調(diào)用其中的equals方法。

但是C#是支持的，所以，會判斷元類型的Type，然后選取對應(yīng)的Equals方法。

?

現(xiàn)在回頭看下RemoveAt方法，該方法仍然會調(diào)用Array.Copy操作，所以，可想而知刪除操作的復(fù)雜度了，內(nèi)存中平均刪除一個元素，要移動n/2個元素，復(fù)雜度為O(n)

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而RemoveAll方法本身是復(fù)雜度為O(n)的，所以最好不要在循環(huán)中寫Remove操作吧。

轉(zhuǎn)載于:https://www.cnblogs.com/mcyushao/p/10629611.html

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