public class ConcurrentBag<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T>
{
    // ThreadLocalList對象包含每個線程的數(shù)據(jù)
    ThreadLocal<ThreadLocalList> m_locals;

    // 這個頭指針和尾指針指向中的第一個和最后一個本地列表，這些本地列表分散在不同線程中
    // 允許在線程局部對象上枚舉
    volatile ThreadLocalList m_headList, m_tailList;

    // 這個標志是告知操作線程必須同步操作
    // 在GlobalListsLock 鎖中 設(shè)置
    bool m_needSync;
}

[Serializable]
// 構(gòu)造了雙向鏈表的節(jié)點
internal class Node
{
    public Node(T value)
    {
        m_value = value;
    }
    public readonly T m_value;
    public Node m_next;
    public Node m_prev;
}

/// <summary>
/// 集合操作類型
/// </summary>
internal enum ListOperation
{
    None,
    Add,
    Take
};

/// <summary>
/// 線程鎖定的類
/// </summary>
internal class ThreadLocalList
{
    // 雙向鏈表的頭結(jié)點 如果為null那么表示鏈表為空
    internal volatile Node m_head;

    // 雙向鏈表的尾節(jié)點
    private volatile Node m_tail;

    // 定義當(dāng)前對List進行操作的種類 
    // 與前面的 ListOperation 相對應(yīng)
    internal volatile int m_currentOp;

    // 這個列表元素的計數(shù)
    private int m_count;

    // The stealing count
    // 這個不是特別理解 好像是在本地列表中 刪除某個Node 以后的計數(shù)
    internal int m_stealCount;

    // 下一個列表 可能會在其它線程中
    internal volatile ThreadLocalList m_nextList;

    // 設(shè)定鎖定是否已進行
    internal bool m_lockTaken;

    // The owner thread for this list
    internal Thread m_ownerThread;

    // 列表的版本，只有當(dāng)列表從空變?yōu)榉强战y(tǒng)計是底層
    internal volatile int m_version;

    /// <summary>
    /// ThreadLocalList 構(gòu)造器
    /// </summary>
    /// <param name="ownerThread">擁有這個集合的線程</param>
    internal ThreadLocalList(Thread ownerThread)
    {
        m_ownerThread = ownerThread;
    }
    /// <summary>
    /// 添加一個新的item到鏈表首部
    /// </summary>
    /// <param name="item">The item to add.</param>
    /// <param name="updateCount">是否更新計數(shù).</param>
    internal void Add(T item, bool updateCount)
    {
        checked
        {
            m_count++;
        }
        Node node = new Node(item);
        if (m_head == null)
        {
            Debug.Assert(m_tail == null);
            m_head = node;
            m_tail = node;
            m_version++; // 因為進行初始化了，所以將空狀態(tài)改為非空狀態(tài)
        }
        else
        {
            // 使用頭插法 將新的元素插入鏈表
            node.m_next = m_head;
            m_head.m_prev = node;
            m_head = node;
        }
        if (updateCount) // 更新計數(shù)以避免此添加同步時溢出
        {
            m_count = m_count - m_stealCount;
            m_stealCount = 0;
        }
    }

    /// <summary>
    /// 從列表的頭部刪除一個item
    /// </summary>
    /// <param name="result">The removed item</param>
    internal void Remove(out T result)
    {
        // 雙向鏈表刪除頭結(jié)點數(shù)據(jù)的流程
        Debug.Assert(m_head != null);
        Node head = m_head;
        m_head = m_head.m_next;
        if (m_head != null)
        {
            m_head.m_prev = null;
        }
        else
        {
            m_tail = null;
        }
        m_count--;
        result = head.m_value;

    }

    /// <summary>
    /// 返回列表頭部的元素
    /// </summary>
    /// <param name="result">the peeked item</param>
    /// <returns>True if succeeded, false otherwise</returns>
    internal bool Peek(out T result)
    {
        Node head = m_head;
        if (head != null)
        {
            result = head.m_value;
            return true;
        }
        result = default(T);
        return false;
    }

    /// <summary>
    /// 從列表的尾部獲取一個item
    /// </summary>
    /// <param name="result">the removed item</param>
    /// <param name="remove">remove or peek flag</param>
    internal void Steal(out T result, bool remove)
    {
        Node tail = m_tail;
        Debug.Assert(tail != null);
        if (remove) // Take operation
        {
            m_tail = m_tail.m_prev;
            if (m_tail != null)
            {
                m_tail.m_next = null;
            }
            else
            {
                m_head = null;
            }
            // Increment the steal count
            m_stealCount++;
        }
        result = tail.m_value;
    }


    /// <summary>
    /// 獲取總計列表計數(shù), 它不是線程安全的, 如果同時調(diào)用它, 則可能提供不正確的計數(shù)
    /// </summary>
    internal int Count
    {
        get
        {
            return m_count - m_stealCount;
        }
    }
}

/// <summary>
/// 嘗試獲取無主列表，無主列表是指線程已經(jīng)被暫?；蛘呓K止，但是集合中的部分數(shù)據(jù)還存儲在那里
/// 這是避免內(nèi)存泄漏的方法
/// </summary>
/// <returns></returns>
private ThreadLocalList GetUnownedList()
{
    //此時必須持有全局鎖
    Contract.Assert(Monitor.IsEntered(GlobalListsLock));

    // 從頭線程列表開始枚舉 找到那些已經(jīng)被關(guān)閉的線程
    // 將它所在的列表對象 返回
    ThreadLocalList currentList = m_headList;
    while (currentList != null)
    {
        if (currentList.m_ownerThread.ThreadState == System.Threading.ThreadState.Stopped)
        {
            currentList.m_ownerThread = Thread.CurrentThread; // the caller should acquire a lock to make this line thread safe
            return currentList;
        }
        currentList = currentList.m_nextList;
    }
    return null;
}
/// <summary>
/// 本地幫助方法，通過線程對象檢索線程線程本地列表
/// </summary>
/// <param name="forceCreate">如果列表不存在，那么創(chuàng)建新列表</param>
/// <returns>The local list object</returns>
private ThreadLocalList GetThreadList(bool forceCreate)
{
    ThreadLocalList list = m_locals.Value;

    if (list != null)
    {
        return list;
    }
    else if (forceCreate)
    {
        // 獲取用于更新操作的 m_tailList 鎖
        lock (GlobalListsLock)
        {
            // 如果頭列表等于空，那么說明集合中還沒有元素
            // 直接創(chuàng)建一個新的
            if (m_headList == null)
            {
                list = new ThreadLocalList(Thread.CurrentThread);
                m_headList = list;
                m_tailList = list;
            }
            else
            {
			   // ConcurrentBag內(nèi)的數(shù)據(jù)是以雙向鏈表的形式分散存儲在各個線程的本地區(qū)域中
                // 通過下面這個方法 可以找到那些存儲有數(shù)據(jù) 但是已經(jīng)被停止的線程
                // 然后將已停止線程的數(shù)據(jù) 移交到當(dāng)前線程管理
                list = GetUnownedList();
                // 如果沒有 那么就新建一個列表 然后更新尾指針的位置
                if (list == null)
                {
                    list = new ThreadLocalList(Thread.CurrentThread);
                    m_tailList.m_nextList = list;
                    m_tailList = list;
                }
            }
            m_locals.Value = list;
        }
    }
    else
    {
        return null;
    }
    Debug.Assert(list != null);
    return list;
}
/// <summary>
/// Adds an object to the <see cref="ConcurrentBag{T}"/>.
/// </summary>
/// <param name="item">The object to be added to the
/// <see cref="ConcurrentBag{T}"/>. The value can be a null reference
/// (Nothing in Visual Basic) for reference types.</param>
public void Add(T item)
{
    // 獲取該線程的本地列表, 如果此線程不存在, 則創(chuàng)建一個新列表 (第一次調(diào)用 add)
    ThreadLocalList list = GetThreadList(true);
    // 實際的數(shù)據(jù)添加操作 在AddInternal中執(zhí)行
    AddInternal(list, item);
}

/// <summary>
/// </summary>
/// <param name="list"></param>
/// <param name="item"></param>
private void AddInternal(ThreadLocalList list, T item)
{
    bool lockTaken = false;
    try
    {
		#pragma warning disable 0420
        Interlocked.Exchange(ref list.m_currentOp, (int)ListOperation.Add);
		#pragma warning restore 0420
	    // 同步案例:
        // 如果列表計數(shù)小于兩個, 因為是雙向鏈表的關(guān)系 為了避免與任何竊取線程發(fā)生沖突 必須獲取鎖
        // 如果設(shè)置了 m_needSync, 這意味著有一個線程需要凍結(jié)包 也必須獲取鎖
        if (list.Count < 2 || m_needSync)
        {
            // 將其重置為None 以避免與竊取線程的死鎖
            list.m_currentOp = (int)ListOperation.None;
            // 鎖定當(dāng)前對象
            Monitor.Enter(list, ref lockTaken);
        }
        // 調(diào)用 ThreadLocalList.Add方法 將數(shù)據(jù)添加到雙向鏈表中
        // 如果已經(jīng)鎖定 那么說明線程安全  可以更新Count 計數(shù)
        list.Add(item, lockTaken);
    }
    finally
    {
        list.m_currentOp = (int)ListOperation.None;
        if (lockTaken)
        {
            Monitor.Exit(list);
        }
    }
}

/// <summary>
/// 本地幫助器方法釋放所有本地列表鎖
/// </summary>
private void ReleaseAllLocks()
{
    // 該方法用于在執(zhí)行線程同步以后 釋放掉所有本地鎖
    // 通過遍歷每個線程中存儲的 ThreadLocalList對象 釋放所占用的鎖
    ThreadLocalList currentList = m_headList;
    while (currentList != null)
    {

        if (currentList.m_lockTaken)
        {
            currentList.m_lockTaken = false;
            Monitor.Exit(currentList);
        }
        currentList = currentList.m_nextList;
    }
}

/// <summary>
/// 從凍結(jié)狀態(tài)解凍包的本地幫助器方法
/// </summary>
/// <param name="lockTaken">The lock taken result from the Freeze method</param>
private void UnfreezeBag(bool lockTaken)
{
    // 首先釋放掉 每個線程中 本地變量的鎖
    // 然后釋放全局鎖
    ReleaseAllLocks();
    m_needSync = false;
    if (lockTaken)
    {
        Monitor.Exit(GlobalListsLock);
    }
}

/// <summary>
/// 本地幫助器函數(shù)等待所有未同步的操作
/// </summary>
private void WaitAllOperations()
{
    Contract.Assert(Monitor.IsEntered(GlobalListsLock));

    ThreadLocalList currentList = m_headList;
    // 自旋等待 等待其它操作完成
    while (currentList != null)
    {
        if (currentList.m_currentOp != (int)ListOperation.None)
        {
            SpinWait spinner = new SpinWait();
            // 有其它線程進行操作時，會將cuurentOp 設(shè)置成 正在操作的枚舉
            while (currentList.m_currentOp != (int)ListOperation.None)
            {
                spinner.SpinOnce();
            }
        }
        currentList = currentList.m_nextList;
    }
}

/// <summary>
/// 本地幫助器方法獲取所有本地列表鎖
/// </summary>
private void AcquireAllLocks()
{
    Contract.Assert(Monitor.IsEntered(GlobalListsLock));

    bool lockTaken = false;
    ThreadLocalList currentList = m_headList;
    
    // 遍歷每個線程的ThreadLocalList 然后獲取對應(yīng)ThreadLocalList的鎖
    while (currentList != null)
    {
        // 嘗試/最后 bllock 以避免在獲取鎖和設(shè)置所采取的標志之間的線程港口
        try
        {
            Monitor.Enter(currentList, ref lockTaken);
        }
        finally
        {
            if (lockTaken)
            {
                currentList.m_lockTaken = true;
                lockTaken = false;
            }
        }
        currentList = currentList.m_nextList;
    }
}

/// <summary>
/// Local helper method to freeze all bag operations, it
/// 1- Acquire the global lock to prevent any other thread to freeze the bag, and also new new thread can be added
/// to the dictionary
/// 2- Then Acquire all local lists locks to prevent steal and synchronized operations
/// 3- Wait for all un-synchronized operations to be done
/// </summary>
/// <param name="lockTaken">Retrieve the lock taken result for the global lock, to be passed to Unfreeze method</param>
private void FreezeBag(ref bool lockTaken)
{
    Contract.Assert(!Monitor.IsEntered(GlobalListsLock));

    // 全局鎖定可安全地防止多線程調(diào)用計數(shù)和損壞 m_needSync
    Monitor.Enter(GlobalListsLock, ref lockTaken);

    // 這將強制同步任何將來的添加/執(zhí)行操作
    m_needSync = true;

    // 獲取所有列表的鎖
    AcquireAllLocks();

    // 等待所有操作完成
    WaitAllOperations();
}

/// <summary>
/// 本地幫助器函數(shù)返回列表中的包項, 這主要由 CopyTo 和 ToArray 使用。
/// 這不是線程安全, 應(yīng)該被稱為凍結(jié)/解凍袋塊
/// 本方法是私有的 只有使用 Freeze/UnFreeze之后才是安全的 
/// </summary>
/// <returns>List the contains the bag items</returns>
private List<T> ToList()
{
    Contract.Assert(Monitor.IsEntered(GlobalListsLock));
	// 創(chuàng)建一個新的List
    List<T> list = new List<T>();
    ThreadLocalList currentList = m_headList;
    // 遍歷每個線程中的ThreadLocalList 將里面的Node的數(shù)據(jù) 添加到list中
    while (currentList != null)
    {
        Node currentNode = currentList.m_head;
        while (currentNode != null)
        {
            list.Add(currentNode.m_value);
            currentNode = currentNode.m_next;
        }
        currentList = currentList.m_nextList;
    }

    return list;
}

/// <summary>
/// Returns an enumerator that iterates through the <see
/// cref="ConcurrentBag{T}"/>.
/// </summary>
/// <returns>An enumerator for the contents of the <see
/// cref="ConcurrentBag{T}"/>.</returns>
/// <remarks>
/// The enumeration represents a moment-in-time snapshot of the contents
/// of the bag.  It does not reflect any updates to the collection after 
/// <see cref="GetEnumerator"/> was called.  The enumerator is safe to use
/// concurrently with reads from and writes to the bag.
/// </remarks>
public IEnumerator<T> GetEnumerator()
{
    // Short path if the bag is empty
    if (m_headList == null)
        return new List<T>().GetEnumerator(); // empty list

    bool lockTaken = false;
    try
    {
        // 首先凍結(jié)整個 ConcurrentBag集合
        FreezeBag(ref lockTaken);
        // 然后ToList 再拿到 List的 IEnumerator
        return ToList().GetEnumerator();
    }
    finally
    {
        UnfreezeBag(lockTaken);
    }
}