.net框架中的linklist，实现的是双向链表，总结下它的实现源码。
先看下LinkedList提供的公有属性和方法的导图：
1 LinkedList实现的接口：
public class LinkedList: ICollection , ICollection, IReadOnlyCollection , ISerializable, IDeserializationCallback
2 LinkedList的全局变量包括，
head是封装的类内头节点；
// This LinkedList is a doubly-Linked circular list.
internal LinkedListNode head;
internal int count;
internal int version;
private object _syncRoot;
//A temporary variable which we need during deserialization.
private SerializationInfo _siInfo;
// names for serialization
private const string VersionName = "Version";
private const string CountName = "Count";
private const string ValuesName = "Data"; 封装的每个节点的数据结构为：
public sealed class LinkedListNode{ public LinkedListNode(T value); //获取LinkedListNode所属的LinkedList public LinkedList List { get; } public LinkedListNode Next { get; } public LinkedListNode Previous { get; } //获取节点中包含的值。 public T Value { get; set; } }
3 构造函数：
public LinkedList() //默认的构造函数
{
} //带有参数的
public LinkedList(IEnumerable collection)
{ if (collection == null)
{ throw new ArgumentNullException(nameof(collection));
} foreach (T item in collection)
{
AddLast(item);
}
} 在构造IEnumerable类型的collection时，用到了AddLast(T)方法，它还有一个重载，工作细节如下：
public LinkedListNodeAddLast(T value) { LinkedListNode result = new LinkedListNode (this, value); if (head == null) { InternalInsertNodeToEmptyList(result); } else { InternalInsertNodeBefore(head, result); } return result; } public void AddLast(LinkedListNode node) { ValidateNewNode(node); if (head == null) { InternalInsertNodeToEmptyList(node); } else { InternalInsertNodeBefore(head, node); } node.list = this; //结合LinkedListNode看 }
以上2个方法，语义是插入某个节点，
分插入新节点到空list中，InternalInsertNodeToEmptyList
插入新节点到不为空的list中，InternalInsertNodeBefore，并且给出在哪个节点前插入newNode，还判断了新插入的节点是不是一个有效的新节点。
internal void ValidateNewNode(LinkedListNodenode) { if (node == null) { throw new ArgumentNullException(nameof(node)); } if (node.list != null) { throw new InvalidOperationException(SR.LinkedListNodeIsAttached); } }
同时，还给出判断一个节点是不是有效节点：
internal void ValidateNode(LinkedListNodenode) { if (node == null) { throw new ArgumentNullException(nameof(node)); } if (node.list != this) { throw new InvalidOperationException(SR.ExternalLinkedListNode); } }
这是双向链表比较重要的内部方法，
InternalInsertNodeToEmptyList的实现细节：
private void InternalInsertNodeToEmptyList(LinkedListNodenewNode) { Debug.Assert(head == null && count == 0, "LinkedList must be empty when this method is called!"); newNode.next = newNode; newNode.prev = newNode; head = newNode; version++; count++; }
InternalInsertNodeBefore的实现细节：
private void InternalInsertNodeBefore(LinkedListNodenode, LinkedListNode newNode) { newNode.next = node; newNode.prev = node.prev; node.prev.next = newNode; node.prev = newNode; version++; count++; }
4 链表自然离不开插入某个节点的公有方法，
public LinkedListNodeAddAfter(LinkedListNode node, T value) { ValidateNode(node); LinkedListNode result = new LinkedListNode (node.list, value); InternalInsertNodeBefore(node.next, result); return result; } public void AddAfter(LinkedListNode node, LinkedListNode newNode) { ValidateNode(node); ValidateNewNode(newNode); InternalInsertNodeBefore(node.next, newNode); newNode.list = this; } public LinkedListNode AddBefore(LinkedListNode node, T value) { ValidateNode(node); LinkedListNode result = new LinkedListNode (node.list, value); InternalInsertNodeBefore(node, result); if (node == head) { head = result; } return result; } public void AddBefore(LinkedListNode node, LinkedListNode newNode) { ValidateNode(node); ValidateNewNode(newNode); InternalInsertNodeBefore(node, newNode); newNode.list = this; if (node == head) { head = newNode; } } public LinkedListNode AddFirst(T value) { LinkedListNode result = new LinkedListNode (this, value); if (head == null) { InternalInsertNodeToEmptyList(result); } else { InternalInsertNodeBefore(head, result); head = result; } return result; } public void AddFirst(LinkedListNode node) { ValidateNewNode(node); if (head == null) { InternalInsertNodeToEmptyList(node); } else { InternalInsertNodeBefore(head, node); head = node; } node.list = this; } public LinkedListNode AddLast(T value) { LinkedListNode result = new LinkedListNode (this, value); if (head == null) { InternalInsertNodeToEmptyList(result); } else { InternalInsertNodeBefore(head, result); } return result; } public void AddLast(LinkedListNode node) { ValidateNewNode(node); if (head == null) { InternalInsertNodeToEmptyList(node); } else { InternalInsertNodeBefore(head, node); } node.list = this; }
5 再看下，清除链表所有节点，此处是设置所有节点不在指向内存堆，然后等GC回收，
public void Clear()
{
LinkedListNode current = head;
while (current != null)
{
LinkedListNode temp = current;
current = current.Next;
// use Next the instead of "next", otherwise it will loop forever
temp.Invalidate();
}
head = null;
count = 0;
version++;
} 6 与只相对应的是移除某个节点的一些列接口，与添加类似，不再赘述，
Clear里面调用了Invalidate()，实现很简单：
internal void Invalidate()
{
list = null;
next = null;
prev = null;
}7 判断某个节点值为value的存在性，里面调用Find方法，
public bool Contains(T value)
{ return Find(value) != null;
}Find方法实现细节，类似的API还有FindLast，因为是双向链表，所以从尾部开始遍历链表即可，
public LinkedListNodeFind(T value) { LinkedListNode node = head; //调用默认相等比较器 EqualityComparer c = EqualityComparer .Default; if (node != null)//链表为null { if (value != null) { do { if (c.Equals(node.item, value)) //Equals：某个节点node的item与value相等 { return node; } node = node.next; } while (node != head); } else { do { if (node.item == null) { return node; } node = node.next; } while (node != head); } } return null; //链表为null，直接返回null }
8 再看一个复制数据到数组的实现：
public void CopyTo(T[] array, int index)
{
if (array == null)
{
throw new ArgumentNullException(nameof(array));
}
if (index < 0)
{
throw new ArgumentOutOfRangeException(nameof(index), index, SR.ArgumentOutOfRange_NeedNonNegNum);
}
if (index > array.Length)
{
throw new ArgumentOutOfRangeException(nameof(index), index, SR.ArgumentOutOfRange_BiggerThanCollection);
}
if (array.Length - index < Count)
{
throw new ArgumentException(SR.Arg_InsufficientSpace);
}
LinkedListNode node = head;
if (node != null)
{
do
{
array[index++] = node.item;
node = node.next;
} while (node != head); //双向链表，再次遍历到头结点时
}
} 
