Core源码（九）List

Core里面的List沿用的framework的结构，整个core的体系是把项目的组织方式改变了，变得更灵活，更可配置，不像framework那么死板。但底层的数据结构和中间代码CLR并没有很大的改变。

在framework源码中的位置

 

无参构造函数和变量

首先我们来看构造函数和一些内部的变量，然后默认的_defaultCapacity数组长度是4，构造函数上也有说明，一旦加入元素，capacity会增加到16，然后以2倍的乘数增长。

List的内部是一个私有的T[] _items对象，在无参的构造函数中，可以看到_items = _emptyArray，也就是被赋值了静态的空数组，这算是个小技巧，List实例的很多方法都涉及到空数组参数，这样设置一个静态只读的空数组，大家都可以共用这个对象。如果一旦被插入数据，或者修改数组的Capacity就会进行再次实例化到对应Capacity。

_size记录的是当前数组内，有赋值的数量，也就是我们用的Count属性，数组的实际容量肯定是大于等于_size的。

public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>
{
    private const int _defaultCapacity = 4;
    private T[] _items;
    [ContractPublicPropertyName("Count")]
    private int _size;
    private int _version;
    [NonSerialized]
    private Object _syncRoot;

    static readonly T[]  _emptyArray = new T[0]; 
    // Constructs a List. The list is initially empty and has a capacity
    // of zero. Upon adding the first element to the list the capacity is
    // increased to 16, and then increased in multiples of two as required.
    public List() {
        _items = _emptyArray;
    }
}

IEnumerable<T>的构造函数

// Constructs a List, copying the contents of the given collection. The
// size and capacity of the new list will both be equal to the size of the
// given collection.
public List(IEnumerable<T> collection) {
    if (collection==null)
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);

    ICollection<T> c = collection as ICollection<T>;
    if( c != null) {
        int count = c.Count;
        if (count == 0)
        {
            _items = _emptyArray;
        }
        else {
            _items = new T[count];
            c.CopyTo(_items, 0);
            _size = count;
        }
    }    
    else {                
        _size = 0;
        _items = _emptyArray;
        // This enumerable could be empty.  Let Add allocate a new array, if needed.
        // Note it will also go to _defaultCapacity first, not 1, then 2, etc.
        using(IEnumerator<T> en = collection.GetEnumerator()) {
            while(en.MoveNext()) {
                Add(en.Current);                                    
            }
        }
    }
}

这里如果集合不为空，设置对应_size变量为目标集合的数量，然后调用CopyTo 方法进行拷贝，CopyTo 内部调用System.Array.Copy,此方法等效于标准 C/C++ 函数 memmove，而不是 memcpy。此方法的运算复杂度为 O(n)，其中 n 是 length。

 

MethodImplAttribute(MethodImplOptions.InternalCall)]用于说明该方法的具体实现可以从CLR内部找到。也就是代码到了这个程度已经脱离了C#范围，是clr内部的实现了。

Capacity属性和Count

Capacity的属性如果用户不手动设置，一般会使用EnsureCapacity来自动的根据集合大小进行调整。

public int Capacity 
{
    get {
        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;
            }
        }
    }
}
// Read-only property describing how many elements are in the List.
public int Count {
    get {
        Contract.Ensures(Contract.Result<int>() >= 0);
        return _size; 
    }
}

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;
    }
}

EnsureCapacity方法的参数min是目前能接受的最小容量，就是操作后数组的长度。

       如果目前的数组长度小于min就返回，如果新的newCapacity大于最大数组允许的长度就为最大长度。如果翻倍后还是小于min，那么就把newCapacity设置为min。

按索引获取和修改

从代码可知，使用索引获取和设置都是O(1)的复杂度

public T this[int index] 
{
    get {
        // Following trick can reduce the range check by one
        if ((uint) index >= (uint)_size) {
            ThrowHelper.ThrowArgumentOutOfRangeException();
        }
        Contract.EndContractBlock();
        return _items[index]; 
    }
    set {
        if ((uint) index >= (uint)_size) {
            ThrowHelper.ThrowArgumentOutOfRangeException();
        }
        Contract.EndContractBlock();
        _items[index] = value;
        _version++;
    }
}

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

// Inserts an element into this list at a given index. The size of the list
// is increased by one. If required, the capacity of the list is doubled
// before inserting the new element.
// 
public void Insert(int index, T item) {
    // Note that insertions at the end are legal.
    if ((uint) index > (uint)_size) {
        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_ListInsert);
    }
    Contract.EndContractBlock();
    if (_size == _items.Length) EnsureCapacity(_size + 1);
    if (index < _size) {
        Array.Copy(_items, index, _items, index + 1, _size - index);
    }
    _items[index] = item;
    _size++;            
    _version++;
}

AddRange

AddRange分为默认的和指定索引的AddRange，

// Adds the elements of the given collection to the end of this list. If
// required, the capacity of the list is increased to twice the previous
// capacity or the new size, whichever is larger.
//
public void AddRange(IEnumerable<T> collection) {
    Contract.Ensures(Count >= Contract.OldValue(Count));
    InsertRange(_size, collection);
}

// 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);
    }

    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++;
}

这里的步骤是先确保容量足够，然后调用Array.Copy为将要插入的数据余出空间。然后再次调用把新插入的数组赋值到_items。

List中的Remove

Remove和RemoveRange实际都是调用Array.Copy方法，这里removeAll的循环判断稍微有点绕，但是仔细研究下就能懂。

public bool Remove(T item) {
    int index = IndexOf(item);
    if (index >= 0) {
        RemoveAt(index);
        return true;
    }

    return false;
}
void System.Collections.IList.Remove(Object item)
{
    if(IsCompatibleObject(item)) {            
        Remove((T) item);
    }
}

RemoveAt

// 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();
    }
    _size--;
    if (index < _size) {
        Array.Copy(_items, index + 1, _items, index, _size - index);
//相当于把数组index后面的元素都向前赋值
    }
    _items[_size] = default(T);
    _version++;
}

// Removes a range of elements from this list.
// 
public void RemoveRange(int index, int count) {
    if (index < 0) {
        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
    }

    if (count < 0) {
        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
    }
        
    if (_size - index < count)
        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
    Contract.EndContractBlock();

    if (count > 0) {
        int i = _size;
        _size -= count;
        if (index < _size) {
            Array.Copy(_items, index + count, _items, index, _size - index);
        }
        Array.Clear(_items, _size, count);
        _version++;
    }
}

RemoveAll

// This method removes all items which matches the predicate.
// The complexity is O(n).   
public int RemoveAll(Predicate<T> match) {
    if( match == null) {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    int freeIndex = 0;   // the first free slot in items array

    // Find the first item which needs to be removed.
    while( freeIndex < _size && !match(_items[freeIndex])) freeIndex++;            
    if( freeIndex >= _size) return 0;
    
    int current = freeIndex + 1;
    while( current < _size) {
        // Find the first item which needs to be kept.
        while( current < _size && match(_items[current])) current++;            
        if( current < _size) {
            // copy item to the free slot.
            _items[freeIndex++] = _items[current++];
        }
}

    Array.Clear(_items, freeIndex, _size - freeIndex);
    int result = _size - freeIndex;
    _size = freeIndex;
    _version++;
    return result;
}