ConcurrentDictionary源码概读

　　ConcurrentDictionary的数据结构主要由Tables和Node组成，其中Tables包括桶（Node，节点）数组、局部锁（Local lock）、每个锁保护的元素数量（PerLock）。Node包含用户实际操作的key和value，以及为实现链表数据结构的下一个节点（Next Node）的引用和当前节点key的原始（未取正）散列值。以及其它一些标识。

         private class Tables
         {
             /// <summary>
             /// 每个桶的单链表
             /// </summary>
             internal readonly Node[] m_buckets; 

             /// <summary>
             /// 锁数组，每个锁都锁住table的一部分
             /// </summary>
             internal readonly object[] m_locks;

             /// <summary>
             /// 每个锁保护的元素的数量
             /// </summary>
             internal volatile int[] m_countPerLock; 

             /// <summary>
             /// key的比较器
             /// </summary>
             internal readonly IEqualityComparer<TKey> m_comparer; 

             internal Tables(Node[] buckets, object[] locks, int[] countPerLock, IEqualityComparer<TKey> comparer)
             {
                 m_buckets = buckets;
                 m_locks = locks;
                 m_countPerLock = countPerLock;
                 m_comparer = comparer;
             }
         }

         private class Node
         {
             internal TKey m_key;
             internal TValue m_value;
             internal volatile Node m_next;
             internal int m_hashcode;

             internal Node(TKey key, TValue value, int hashcode, Node next)
             {
                 m_key = key;
                 m_value = value;
                 m_next = next;
                 m_hashcode = hashcode;
             }
         }

　　当new一个ConcurrentDictionary时，默认调用无参构造函数，给定默认的并发数量（Environment.ProcessorCount）、默认的键比较器、默认的容量（桶的初始容量，为31），该容量是经过权衡得到，不能被最小的素数整除。之后再处理容量与并发数的关系、容量与锁的关系以及每个锁的最大元素数。将桶、锁对象、锁保护封装在一个对象中，并初始化。

         //初始化 ConcurrentDictionary 类的新实例，
         //该实例为空，具有默认的并发级别和默认的初始容量，并为键类型使用默认比较器。
         public ConcurrentDictionary() :
             this(DefaultConcurrencyLevel, DEFAULT_CAPACITY, true, EqualityComparer<TKey>.Default) { }

         /// <summary>
         /// 无参构造函数真正调用的函数
         /// </summary>
         /// <param name="concurrencyLevel">并发线程的可能数量（更改字典的线程可能数量）</param>
         /// <param name="capacity">容量</param>
         /// <param name="growLockArray">是否动态增加 striped lock 的大小</param>
         /// <param name="comparer">比较器</param>
         internal ConcurrentDictionary(int concurrencyLevel, int capacity, bool growLockArray, IEqualityComparer<TKey> comparer)
         {
             if (concurrencyLevel < )
             {
                 throw new ArgumentOutOfRangeException("concurrencyLevel", GetResource("ConcurrentDictionary_ConcurrencyLevelMustBePositive"));
             }
             if (capacity < )
             {
                 throw new ArgumentOutOfRangeException("capacity", GetResource("ConcurrentDictionary_CapacityMustNotBeNegative"));
             }
             if (comparer == null) throw new ArgumentNullException("comparer");

             //容量应当至少与并发数一致，否则会有锁对象浪费
             if (capacity < concurrencyLevel)
             {
                 capacity = concurrencyLevel;
             }

             //锁对象数组，大小为 并发线程的可能数量
             object[] locks = new object[concurrencyLevel];
             for (int i = ; i < locks.Length; i++)
             {
                 locks[i] = new object();
             }

             //每个锁保护的元素的数量
             int[] countPerLock = new int[locks.Length];
             //单链表中的节点，表示特定的哈希存储桶（桶：Node类型的数组）。
             Node[] buckets = new Node[capacity];
             //可以保持字典内部状态的表，将桶、锁对象、锁保护封装在一个对象中，以便一次原子操作
             m_tables = new Tables(buckets, locks, countPerLock, comparer);
             //是否动态增加 striped lock 的大小
             m_growLockArray = growLockArray;
             //在调整大小操作被触发之前，每个锁可锁住的最大（预计）元素数
             //默认按锁个数平均分配，即Node总个数除以锁总个数
             m_budget = buckets.Length / locks.Length;
         }

　　当调用TryAdd时，实际调用的是内部公共方法TryAddInternal。如果存在key，则始终返回false，如果updateIfExists为true，则更新value，如果不存在key，则始终返回true，并且添加value。详细解读见代码。

         /// <summary>
         /// 尝试将指定的键和值添加到字典
         /// </summary>
         /// <param name="key">要添加的元素的键</param>
         /// <param name="value">要添加的元素的值。对于引用类型，该值可以是空引用</param>
         /// <returns>键值对添加成功则返回true，否则false</returns>
         /// 异常:
         //   T:System.ArgumentNullException:
         //     key 为 null。
         //   T:System.OverflowException:
         //     字典中已包含元素的最大数量(System.Int32.MaxValue)。
         public bool TryAdd(TKey key, TValue value)
         {
             if (key == null) throw new ArgumentNullException("key");
             TValue dummy;
             return TryAddInternal(key, value, false, true, out dummy);
         }

         /// <summary>
         /// 对字典添加和修改的内部公共方法
         /// 如果存在key，则始终返回false，如果updateIfExists为true，则更新value
         /// 如果不存在key，则始终返回true，并且添加value
         /// </summary>
         [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
         private bool TryAddInternal(TKey key, TValue value, bool updateIfExists, bool acquireLock, out TValue resultingValue)
         {
             while (true)
             {
                 //桶序号（下标），锁序号（下标）
                 int bucketNo, lockNo;
                 int hashcode;

                 Tables tables = m_tables;
                 IEqualityComparer<TKey> comparer = tables.m_comparer;
                 hashcode = comparer.GetHashCode(key);

                 //获取桶下标、锁下标
                 GetBucketAndLockNo(hashcode, out bucketNo, out lockNo, tables.m_buckets.Length, tables.m_locks.Length);

                 bool resizeDesired = false;
                 bool lockTaken = false;
 #if FEATURE_RANDOMIZED_STRING_HASHING
 #if !FEATURE_CORECLR
                 bool resizeDueToCollisions = false;
 #endif // !FEATURE_CORECLR
 #endif

                 try
                 {
                     if (acquireLock)
                         //根据上面得到的锁的下标（lockNo），获取对应（lockNo）的对象锁
                         //hash落在不同的锁对象上，因此不同线程获取锁的对象可能不同，降低了“抢锁”概率
                         Monitor.Enter(tables.m_locks[lockNo], ref lockTaken);

                     //在这之前如果tables被修改则有可能未正确锁定，此时需要重试
                     if (tables != m_tables)
                     {
                         continue;
                     }

 #if FEATURE_RANDOMIZED_STRING_HASHING
 #if !FEATURE_CORECLR
                     int collisionCount = ;
 #endif // !FEATURE_CORECLR
 #endif

                     // Try to find this key in the bucket
                     Node prev = null;
                     for (Node node = tables.m_buckets[bucketNo]; node != null; node = node.m_next)
                     {
                         Assert((prev == null && node == tables.m_buckets[bucketNo]) || prev.m_next == node);
                         //如果key已经存在
                         if (comparer.Equals(node.m_key, key))
                         {
                             //如果允许更新，则更新该键值对的值
                             if (updateIfExists)
                             {
                                 //如果可以原子操作则直接赋值
                                 if (s_isValueWriteAtomic)
                                 {
                                     node.m_value = value;
                                 }
                                 //否则需要为更新创建一个新的节点，以便支持不能以原子方式写的类型，
                                 //因为无锁读取也可能在此时发生
                                 else
                                 {
                                     //node.m_next 新节点指向下一个节点
                                     Node newNode = new Node(node.m_key, value, hashcode, node.m_next);
                                     if (prev == null)
                                     {
                                         tables.m_buckets[bucketNo] = newNode;
                                     }
                                     else
                                     {
                                         //上一个节点指向新节点。此时完成单链表的新旧节点替换
                                         prev.m_next = newNode;
                                     }
                                 }
                                 resultingValue = value;
                             }
                             else
                             {
                                 resultingValue = node.m_value;
                             }
                             return false;
                         }
                         //循环到最后时，prev是最后一个node（node.m_next==null）
                         prev = node;

 #if FEATURE_RANDOMIZED_STRING_HASHING
 #if !FEATURE_CORECLR
                         collisionCount++;
 #endif // !FEATURE_CORECLR
 #endif
                     }

 #if FEATURE_RANDOMIZED_STRING_HASHING
 #if !FEATURE_CORECLR
                     if(collisionCount > HashHelpers.HashCollisionThreshold && HashHelpers.IsWellKnownEqualityComparer(comparer))
                     {
                         resizeDesired = true;
                         resizeDueToCollisions = true;
                     }
 #endif // !FEATURE_CORECLR
 #endif

                     //使用可变内存操作插入键值对
                     Volatile.Write<Node>(ref tables.m_buckets[bucketNo], new Node(key, value, hashcode, tables.m_buckets[bucketNo]));
                     checked
                     {
                         //第lockNo个锁保护的元素数量，并检查是否益处
                         tables.m_countPerLock[lockNo]++;
                     }

                     //
                     // If the number of elements guarded by this lock has exceeded the budget, resize the bucket table.
                     // It is also possible that GrowTable will increase the budget but won't resize the bucket table.
                     // That happens if the bucket table is found to be poorly utilized due to a bad hash function.
                     //如果第lockNo个锁要锁的元素超出预计，则需要调整
                     if (tables.m_countPerLock[lockNo] > m_budget)
                     {
                         resizeDesired = true;
                     }
                 }
                 finally
                 {
                     if (lockTaken)
                         //释放第lockNo个锁
                         Monitor.Exit(tables.m_locks[lockNo]);
                 }

                 //
                 // The fact that we got here means that we just performed an insertion. If necessary, we will grow the table.
                 //
                 // Concurrency notes:
                 // - Notice that we are not holding any locks at when calling GrowTable. This is necessary to prevent deadlocks.
                 // - As a result, it is possible that GrowTable will be called unnecessarily. But, GrowTable will obtain lock 0
                 //   and then verify that the table we passed to it as the argument is still the current table.
                 //
                 if (resizeDesired)
                 {
 #if FEATURE_RANDOMIZED_STRING_HASHING
 #if !FEATURE_CORECLR
                     if (resizeDueToCollisions)
                     {
                         GrowTable(tables, (IEqualityComparer<TKey>)HashHelpers.GetRandomizedEqualityComparer(comparer), true, m_keyRehashCount);
                     }
                     else
 #endif // !FEATURE_CORECLR
                     {
                         GrowTable(tables, tables.m_comparer, false, m_keyRehashCount);
                     }
 #else
                     GrowTable(tables, tables.m_comparer, false, m_keyRehashCount);
 #endif
                 }

                 resultingValue = value;
                 return true;
             }
         }

　　需要特别指出的是ConcurrentDictionary在插入、更新、获取键值对时对key的比较默认是使用的引用比较，不同于Dictionary使用引用加散列值。在Dictionary中，只有两者都一致才相等，ConcurrentDictionary则只判断引用相等。前提是未重写Equals。

         /// <summary>
         /// Attempts to get the value associated with the specified key from the <see
         /// cref="ConcurrentDictionary{TKey,TValue}"/>.
         /// </summary>
         /// <param name="key">The key of the value to get.</param>
         /// <param name="value">When this method returns, <paramref name="value"/> contains the object from
         /// the
         /// <see cref="ConcurrentDictionary{TKey,TValue}"/> with the specified key or the default value of
         /// <typeparamref name="TValue"/>, if the operation failed.</param>
         /// <returns>true if the key was found in the <see cref="ConcurrentDictionary{TKey,TValue}"/>;
         /// otherwise, false.</returns>
         /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
         /// (Nothing in Visual Basic).</exception>
         [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
         public bool TryGetValue(TKey key, out TValue value)
         {
             if (key == null) throw new ArgumentNullException("key");

             int bucketNo, lockNoUnused;

             // We must capture the m_buckets field in a local variable. It is set to a new table on each table resize.
             Tables tables = m_tables;
             IEqualityComparer<TKey> comparer = tables.m_comparer;
             GetBucketAndLockNo(comparer.GetHashCode(key), out bucketNo, out lockNoUnused, tables.m_buckets.Length, tables.m_locks.Length);

             // We can get away w/out a lock here.
             // The Volatile.Read ensures that the load of the fields of 'n' doesn't move before the load from buckets[i].
             Node n = Volatile.Read<Node>(ref tables.m_buckets[bucketNo]);

             while (n != null)
             {
                 //默认比较的是引用
                 if (comparer.Equals(n.m_key, key))
                 {
                     value = n.m_value;
                     return true;
                 }
                 n = n.m_next;
             }

             value = default(TValue);
             return false;
         }

　　其它一些需要知道的内容，比如默认并发数、如何为指定key计算桶号和锁号等

 #if !FEATURE_CORECLR
         [NonSerialized]
 #endif
         private volatile Tables m_tables; // Internal tables of the dictionary
         // NOTE: this is only used for compat reasons to serialize the comparer.
         // This should not be accessed from anywhere else outside of the serialization methods.
         internal IEqualityComparer<TKey> m_comparer;
 #if !FEATURE_CORECLR
         [NonSerialized]
 #endif
         private readonly bool m_growLockArray; // Whether to dynamically increase the size of the striped lock

         // How many times we resized becaused of collisions.
         // This is used to make sure we don't resize the dictionary because of multi-threaded Add() calls
         // that generate collisions. Whenever a GrowTable() should be the only place that changes this
 #if !FEATURE_CORECLR
         // The field should be have been marked as NonSerialized but because we shipped it without that attribute in 4.5.1.
         // we can't add it back without breaking compat. To maximize compat we are going to keep the OptionalField attribute
         // This will prevent cases where the field was not serialized.
         [OptionalField]
 #endif
         private int m_keyRehashCount;

 #if !FEATURE_CORECLR
         [NonSerialized]
 #endif
         private int m_budget; // The maximum number of elements per lock before a resize operation is triggered

 #if !FEATURE_CORECLR // These fields are not used in CoreCLR
         private KeyValuePair<TKey, TValue>[] m_serializationArray; // Used for custom serialization

         private int m_serializationConcurrencyLevel; // used to save the concurrency level in serialization

         private int m_serializationCapacity; // used to save the capacity in serialization
 #endif

         // The default capacity, i.e. the initial # of buckets. When choosing this value, we are making
         // a trade-off between the size of a very small dictionary, and the number of resizes when
         // constructing a large dictionary. Also, the capacity should not be divisible by a small prime.
         private const int DEFAULT_CAPACITY = ;

         // The maximum size of the striped lock that will not be exceeded when locks are automatically
         // added as the dictionary grows. However, the user is allowed to exceed this limit by passing
         // a concurrency level larger than MAX_LOCK_NUMBER into the constructor.
         private const int MAX_LOCK_NUMBER = ;

         private const int PROCESSOR_COUNT_REFRESH_INTERVAL_MS = ; // How often to refresh the count, in milliseconds.
         private static volatile int s_processorCount; // The last count seen.
         private static volatile int s_lastProcessorCountRefreshTicks; // The last time we refreshed.

         /// <summary>
         /// Gets the number of available processors
         /// </summary>
         private static int ProcessorCount
         {
             get
             {
                 int now = Environment.TickCount;
                 int procCount = s_processorCount;
                 if (procCount ==  || (now - s_lastProcessorCountRefreshTicks) >= PROCESSOR_COUNT_REFRESH_INTERVAL_MS)
                 {
                     s_processorCount = procCount = Environment.ProcessorCount;
                     s_lastProcessorCountRefreshTicks = now;
                 }

                 Contract.Assert(procCount >  && procCount <= ,
                     "Processor count not within the expected range (1 - 64).");

                 return procCount;
             }
         }

         // Whether TValue is a type that can be written atomically (i.e., with no danger of torn reads)
         private static readonly bool s_isValueWriteAtomic = IsValueWriteAtomic();
         /// <summary>
         /// The number of concurrent writes for which to optimize by default.
         /// </summary>
         private static int DefaultConcurrencyLevel
         {
             get { return ProcessorCount; }
         }
         /// <summary>
         /// Replaces the bucket table with a larger one. To prevent multiple threads from resizing the
         /// table as a result of ----s, the Tables instance that holds the table of buckets deemed too
         /// small is passed in as an argument to GrowTable(). GrowTable() obtains a lock, and then checks
         /// the Tables instance has been replaced in the meantime or not.
         /// The <paramref name="rehashCount"/> will be used to ensure that we don't do two subsequent resizes
         /// because of a collision
         /// </summary>
         private void GrowTable(Tables tables, IEqualityComparer<TKey> newComparer, bool regenerateHashKeys, int rehashCount)
         {
             int locksAcquired = ;
             try
             {
                 // The thread that first obtains m_locks[0] will be the one doing the resize operation
                 AcquireLocks(, , ref locksAcquired);

                 if (regenerateHashKeys && rehashCount == m_keyRehashCount)
                 {
                     // This method is called with regenerateHashKeys==true when we detected
                     // more than HashHelpers.HashCollisionThreshold collisions when adding a new element.
                     // In that case we are in the process of switching to another (randomized) comparer
                     // and we have to re-hash all the keys in the table.
                     // We are only going to do this if we did not just rehash the entire table while waiting for the lock
                     tables = m_tables;
                 }
                 else
                 {
                     // If we don't require a regeneration of hash keys we want to make sure we don't do work when
                     // we don't have to
                     if (tables != m_tables)
                     {
                         // We assume that since the table reference is different, it was already resized (or the budget
                         // was adjusted). If we ever decide to do table shrinking, or replace the table for other reasons,
                         // we will have to revisit this logic.
                         return;
                     }

                     // Compute the (approx.) total size. Use an Int64 accumulation variable to avoid an overflow.
                     long approxCount = ;
                     for (int i = ; i < tables.m_countPerLock.Length; i++)
                     {
                         approxCount += tables.m_countPerLock[i];
                     }

                     //
                     // If the bucket array is too empty, double the budget instead of resizing the table
                     //
                     if (approxCount < tables.m_buckets.Length / )
                     {
                         m_budget =  * m_budget;
                         if (m_budget < )
                         {
                             m_budget = int.MaxValue;
                         }

                         return;
                     }
                 }
                 // Compute the new table size. We find the smallest integer larger than twice the previous table size, and not divisible by
                 // 2,3,5 or 7. We can consider a different table-sizing policy in the future.
                 int newLength = ;
                 bool maximizeTableSize = false;
                 try
                 {
                     checked
                     {
                         // Double the size of the buckets table and add one, so that we have an odd integer.
                         newLength = tables.m_buckets.Length *  + ;

                         // Now, we only need to check odd integers, and find the first that is not divisible
                         // by 3, 5 or 7.
                         while (newLength %  ==  || newLength %  ==  || newLength %  == )
                         {
                             newLength += ;
                         }

                         Assert(newLength %  != );

                         if (newLength > Array.MaxArrayLength)
                         {
                             maximizeTableSize = true;
                         }
                     }
                 }
                 catch (OverflowException)
                 {
                     maximizeTableSize = true;
                 }

                 if (maximizeTableSize)
                 {
                     newLength = Array.MaxArrayLength;

                     // We want to make sure that GrowTable will not be called again, since table is at the maximum size.
                     // To achieve that, we set the budget to int.MaxValue.
                     //
                     // (There is one special case that would allow GrowTable() to be called in the future:
                     // calling Clear() on the ConcurrentDictionary will shrink the table and lower the budget.)
                     m_budget = int.MaxValue;
                 }

                 // Now acquire all other locks for the table
                 AcquireLocks(, tables.m_locks.Length, ref locksAcquired);

                 object[] newLocks = tables.m_locks;

                 // Add more locks
                 if (m_growLockArray && tables.m_locks.Length < MAX_LOCK_NUMBER)
                 {
                     newLocks = new object[tables.m_locks.Length * ];
                     Array.Copy(tables.m_locks, newLocks, tables.m_locks.Length);

                     for (int i = tables.m_locks.Length; i < newLocks.Length; i++)
                     {
                         newLocks[i] = new object();
                     }
                 }

                 Node[] newBuckets = new Node[newLength];
                 int[] newCountPerLock = new int[newLocks.Length];

                 // Copy all data into a new table, creating new nodes for all elements
                 for (int i = ; i < tables.m_buckets.Length; i++)
                 {
                     Node current = tables.m_buckets[i];
                     while (current != null)
                     {
                         Node next = current.m_next;
                         int newBucketNo, newLockNo;
                         int nodeHashCode = current.m_hashcode;

                         if (regenerateHashKeys)
                         {
                             // Recompute the hash from the key
                             nodeHashCode = newComparer.GetHashCode(current.m_key);
                         }

                         GetBucketAndLockNo(nodeHashCode, out newBucketNo, out newLockNo, newBuckets.Length, newLocks.Length);

                         newBuckets[newBucketNo] = new Node(current.m_key, current.m_value, nodeHashCode, newBuckets[newBucketNo]);

                         checked
                         {
                             newCountPerLock[newLockNo]++;
                         }

                         current = next;
                     }
                 }

                 // If this resize regenerated the hashkeys, increment the count
                 if (regenerateHashKeys)
                 {
                     // We use unchecked here because we don't want to throw an exception if
                     // an overflow happens
                     unchecked
                     {
                         m_keyRehashCount++;
                     }
                 }

                 // Adjust the budget
                 m_budget = Math.Max(, newBuckets.Length / newLocks.Length);

                 // Replace tables with the new versions
                 m_tables = new Tables(newBuckets, newLocks, newCountPerLock, newComparer);
             }
             finally
             {
                 // Release all locks that we took earlier
                 ReleaseLocks(, locksAcquired);
             }
         }

         /// <summary>
         /// 为指定key计算桶号和锁号
         /// </summary>
         /// <param name="hashcode">key的hashcode</param>
         /// <param name="bucketNo"></param>
         /// <param name="lockNo"></param>
         /// <param name="bucketCount">桶数量</param>
         /// <param name="lockCount">锁数量</param>
         private void GetBucketAndLockNo(int hashcode, out int bucketNo, out int lockNo, int bucketCount, int lockCount)
         {
             //取正hashcode，余数恒小于除数
             bucketNo = (hashcode & 0x7fffffff) % bucketCount;
             //若桶下标与锁个数的余数相同，则这一簇数据都使用同一个锁（局部锁）
             lockNo = bucketNo % lockCount;

             Assert(bucketNo >=  && bucketNo < bucketCount);
             Assert(lockNo >=  && lockNo < lockCount);
         }

         /// <summary>
         /// Determines whether type TValue can be written atomically
         /// </summary>
         private static bool IsValueWriteAtomic()
         {
             Type valueType = typeof(TValue);

             //
             // Section 12.6.6 of ECMA CLI explains which types can be read and written atomically without
             // the risk of tearing.
             //
             // See http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-335.pdf
             //
             if (valueType.IsClass)
             {
                 return true;
             }
             switch (Type.GetTypeCode(valueType))
             {
                 case TypeCode.Boolean:
                 case TypeCode.Byte:
                 case TypeCode.Char:
                 case TypeCode.Int16:
                 case TypeCode.Int32:
                 case TypeCode.SByte:
                 case TypeCode.Single:
                 case TypeCode.UInt16:
                 case TypeCode.UInt32:
                     return true;
                 case TypeCode.Int64:
                 case TypeCode.Double:
                 case TypeCode.UInt64:
                     return IntPtr.Size == ;
                 default:
                     return false;
             }
         }