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C#中ConcurrentDictionary的AddOrUpdate/GetOrAdd的并发问题

news 2026/3/26 19:11:07

这个是偶然发现的，我们看下面的代码：

    public static void Main(){int addCount = 0;int updateCount = 0;var dict = new ConcurrentDictionary<int, int>();for (var i = 0; i < 10; i++){new Thread(() =>{dict.AddOrUpdate(1, _ => Interlocked.Increment(ref addCount), (_, _) => Interlocked.Increment(ref updateCount));}).Start();}Console.WriteLine($"addCount={addCount}");Console.WriteLine($"updateCount={updateCount}");}

你是否会觉得输出addCount=1，updateCount=9？就是addValueFactory只会被调用一次？调用了addValueFactory就不会调用updateValueFactory?

再看下面的代码：

    public static void Main(){int addCount = 0;var dict = new ConcurrentDictionary<int, int>();for (var i = 0; i < 10; i++){new Thread(() =>{dict.GetOrAdd(1, _ => Interlocked.Increment(ref addCount));}).Start();}Console.WriteLine($"addCount={addCount}");}

你是否会觉得输出addCount=1？就是valueFactory只会被调用一次？

如果你这么觉得，那你就错了，同样的，我也被骗了这么多年！！！结果是不确定的！！

微软只说ConcurrentDictionary是线程安全的，却没说它体现在哪里，我们看看AddOrUpdate和GetOrAdd的源码就知道了

AddOrUpdate

    public TValue AddOrUpdate(TKey key, Func<TKey, TValue> addValueFactory, Func<TKey, TValue, TValue> updateValueFactory){if (key is null){ThrowHelper.ThrowKeyNullException();}if (addValueFactory is null){ThrowHelper.ThrowArgumentNullException(nameof(addValueFactory));}if (updateValueFactory is null){ThrowHelper.ThrowArgumentNullException(nameof(updateValueFactory));}IEqualityComparer<TKey>? comparer = _comparer;int hashcode = comparer is null ? key.GetHashCode() : comparer.GetHashCode(key);while (true){if (TryGetValueInternal(key, hashcode, out TValue? oldValue)){// key exists, try to updateTValue newValue = updateValueFactory(key, oldValue);if (TryUpdateInternal(key, hashcode, newValue, oldValue)){return newValue;}}else{// key doesn't exist, try to addif (TryAddInternal(key, hashcode, addValueFactory(key), updateIfExists: false, acquireLock: true, out TValue resultingValue)){return resultingValue;}}}}

GetOrAdd

    public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory){if (key is null){ThrowHelper.ThrowKeyNullException();}if (valueFactory is null){ThrowHelper.ThrowArgumentNullException(nameof(valueFactory));}IEqualityComparer<TKey>? comparer = _comparer;int hashcode = comparer is null ? key.GetHashCode() : comparer.GetHashCode(key);if (!TryGetValueInternal(key, hashcode, out TValue? resultingValue)){TryAddInternal(key, hashcode, valueFactory(key), updateIfExists: false, acquireLock: true, out resultingValue);}return resultingValue;}

它们内部都没有上锁，AddOrUpdate就是在一个while(true)循环内，不停的尝试修改和添加，每次循环或者调用addValueFactory或者updateValueFactory，GetOrAdd获取不到值就会去调用valueFactory，所以，如果你在这些factory里面有并发操作的话，一定要注意！！

那怎么解决这个问题呢，你说在每个地方上锁吧，好像很繁琐，我的解决办法是使用拓展方法来解决：

DictionaryExtensions

    public static class DictionaryExtensions{public static TValue AddOrUpdateValue<TKey, TValue, TArg>(this ConcurrentDictionary<TKey, TValue> dictionary,TKey key, Func<TKey, TArg, TValue> addValueFactory, Func<TKey, TValue, TArg, TValue> updateValueFactory, TArg factoryArgument){bool hasAddValue = false;var addValue = default(TValue);bool hasUpdateValue = false;var updateValue = default(TValue);Func<TKey, TArg, TValue> newAddValueFuctory = (key, arg) =>{if (!hasAddValue){addValue = hasUpdateValue ? updateValue : addValueFactory(key, arg);hasAddValue = true;}return addValue;};Func<TKey, TValue, TArg, TValue> newUpdateValueFactory = (key, value, arg) =>{if (!hasUpdateValue){addValue = hasAddValue ? addValue : updateValueFactory(key, value, arg);hasAddValue = true;}return updateValue;};lock (dictionary){return dictionary.AddOrUpdate(key, newAddValueFuctory, newUpdateValueFactory, factoryArgument);}}public static TValue AddOrUpdateValue<TKey, TValue>(this ConcurrentDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TValue> addValueFactory, Func<TKey, TValue, TValue> updateValueFactory){bool hasAddValue = false;var addValue = default(TValue);bool hasUpdateValue = false;var updateValue = default(TValue);Func<TKey, TValue> newAddValueFuctory = key =>{if (!hasAddValue){addValue = hasUpdateValue ? updateValue : addValueFactory(key);hasAddValue = true;}return addValue;};Func<TKey, TValue, TValue> newUpdateValueFactory = (key, value) =>{if (!hasUpdateValue){addValue = hasAddValue ? addValue : updateValueFactory(key, value);hasUpdateValue = true;}return updateValue;};lock (dictionary){return dictionary.AddOrUpdate(key, newAddValueFuctory, newUpdateValueFactory);}}public static TValue AddOrUpdateValue<TKey, TValue>(this ConcurrentDictionary<TKey, TValue> dictionary, TKey key, TValue addValue, Func<TKey, TValue, TValue> updateValueFactory){bool hasUpdateValue = false;var updateValue = default(TValue);Func<TKey, TValue, TValue> newUpdateValueFactory = (key, value) =>{if (!hasUpdateValue){addValue = updateValueFactory(key, value);hasUpdateValue = true;}return updateValue;};lock (dictionary){return dictionary.AddOrUpdate(key, addValue, newUpdateValueFactory);}}public static TValue GetOrAddValue<TKey, TValue>(this ConcurrentDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TValue> valueFactory){bool hasAddValue = false;var addValue = default(TValue);Func<TKey, TValue> newAddValueFuctory = key =>{if (!hasAddValue){addValue = valueFactory(key);hasAddValue = true;}return addValue;};lock (dictionary){return dictionary.GetOrAdd(key, newAddValueFuctory);}}public static TValue GetOrAddValue<TKey, TValue, TArg>(this ConcurrentDictionary<TKey, TValue> dictionary, TKey key, Func<TKey, TArg, TValue> valueFactory, TArg factoryArgument){bool hasAddValue = false;var addValue = default(TValue);Func<TKey, TArg, TValue> newAddValueFuctory = (key, arg) =>{if (!hasAddValue){addValue = valueFactory(key, arg);hasAddValue = true;}return addValue;};lock (dictionary){return dictionary.GetOrAdd(key, newAddValueFuctory, factoryArgument);}}}

这个是简单的封装，满足我们之前一如既往的认识，向上面的例子，我们把AddOrUpdate和GetOrAdd分表改成AddOrUpdateValue和GetOrAddValue就可以了。

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