c# - 为什么 ConcurrentDictionary.GetOrAdd(key, valueFactory) 允许 valueFactory 被调用两次？

Question

我将并发字典用作线程安全的静态缓存并注意到以下行为:

来自 the MSDN docs on GetOrAdd :

If you call GetOrAdd simultaneously on different threads, addValueFactory may be called multiple times, but its key/value pair might not be added to the dictionary for every call.

我希望能够保证工厂只被调用一次。有没有什么方法可以使用 ConcurrentDictionary API 来执行此操作而无需诉诸我自己的单独同步(例如，锁定在 valueFactory 中)？

我的用例是 valueFactory 在动态模块中生成类型，所以如果同一个键的两个 valueFactory 同时运行，我会点击:

System.ArgumentException:程序集中类型名称重复。

Answer 1

您可以使用这样输入的字典:ConcurrentDictionary<TKey, Lazy<TValue>> ，然后您的值(value)工厂将返回 Lazy<TValue>已使用 LazyThreadSafetyMode.ExecutionAndPublication 初始化的对象，这是 Lazy<TValue> 使用的默认选项如果你不指定它。通过指定 LazyThreadSafetyMode.ExecutionAndPublication你告诉 Lazy 只有一个线程可以初始化和设置对象的值。

这导致 ConcurrentDictionary仅使用 Lazy<TValue> 的一个实例对象，以及 Lazy<TValue>对象保护多个线程免于初始化其值。

即

var dict = new ConcurrentDictionary<int, Lazy<Foo>>();
dict.GetOrAdd(key,  
    (k) => new Lazy<Foo>(valueFactory)
);

缺点是每次访问字典中的对象时都需要调用 *.Value。这里有一些 extensions这会有所帮助。

public static class ConcurrentDictionaryExtensions
{
    public static TValue GetOrAdd<TKey, TValue>(
        this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
        TKey key, Func<TKey, TValue> valueFactory
    )
    {
        return @this.GetOrAdd(key,
            (k) => new Lazy<TValue>(() => valueFactory(k))
        ).Value;
    }

    public static TValue AddOrUpdate<TKey, TValue>(
        this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
        TKey key, Func<TKey, TValue> addValueFactory,
        Func<TKey, TValue, TValue> updateValueFactory
    )
    {
        return @this.AddOrUpdate(key,
            (k) => new Lazy<TValue>(() => addValueFactory(k)),
            (k, currentValue) => new Lazy<TValue>(
                () => updateValueFactory(k, currentValue.Value)
            )
        ).Value;
    }

    public static bool TryGetValue<TKey, TValue>(
        this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
        TKey key, out TValue value
    )
    {
        value = default(TValue);

        var result = @this.TryGetValue(key, out Lazy<TValue> v);

        if (result) value = v.Value;

        return result;
   }

   // this overload may not make sense to use when you want to avoid
   //  the construction of the value when it isn't needed
   public static bool TryAdd<TKey, TValue>(
       this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
       TKey key, TValue value
   )
   {
       return @this.TryAdd(key, new Lazy<TValue>(() => value));
   }

   public static bool TryAdd<TKey, TValue>(
       this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
       TKey key, Func<TKey, TValue> valueFactory
   )
   {
       return @this.TryAdd(key,
           new Lazy<TValue>(() => valueFactory(key))
       );
   }

   public static bool TryRemove<TKey, TValue>(
       this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
       TKey key, out TValue value
   )
   {
       value = default(TValue);

       if (@this.TryRemove(key, out Lazy<TValue> v))
       {
           value = v.Value;
           return true;
       }
       return false;
   }

   public static bool TryUpdate<TKey, TValue>(
       this ConcurrentDictionary<TKey, Lazy<TValue>> @this,
       TKey key, Func<TKey, TValue, TValue> updateValueFactory
   )
   {
       if (!@this.TryGetValue(key, out Lazy<TValue> existingValue))
           return false;

       return @this.TryUpdate(key,
           new Lazy<TValue>(
               () => updateValueFactory(key, existingValue.Value)
           ),
           existingValue
       );
   }
}