Question

正在做一些ObservableCollection实现的研究我发现了一些我无法理解的代码。这是由.Net Reflector反编译的代码片段：

[NonSerialized]
private NotifyCollectionChangedEventHandler CollectionChanged;
[NonSerialized]
private PropertyChangedEventHandler PropertyChanged;

event PropertyChangedEventHandler INotifyPropertyChanged.PropertyChanged
{
  add
  {
    this.PropertyChanged += value;
  }
  remove
  {
    this.PropertyChanged -= value;
  }
}

public virtual event NotifyCollectionChangedEventHandler CollectionChanged
{
  add
  {
    NotifyCollectionChangedEventHandler changedEventHandler = this.CollectionChanged;
    NotifyCollectionChangedEventHandler comparand;
    do
    {
      comparand = changedEventHandler;
      changedEventHandler = Interlocked.CompareExchange<NotifyCollectionChangedEventHandler>(ref this.CollectionChanged, comparand + value, comparand);
    }
    while (changedEventHandler != comparand);
  }
  remove
  {
    NotifyCollectionChangedEventHandler changedEventHandler = this.CollectionChanged;
    NotifyCollectionChangedEventHandler comparand;
    do
    {
      comparand = changedEventHandler;
      changedEventHandler = Interlocked.CompareExchange<NotifyCollectionChangedEventHandler>(ref this.CollectionChanged, comparand - value, comparand);
    }
    while (changedEventHandler != comparand);
  }
}

protected virtual event PropertyChangedEventHandler PropertyChanged
{
  add
  {
    PropertyChangedEventHandler changedEventHandler = this.PropertyChanged;
    PropertyChangedEventHandler comparand;
    do
    {
      comparand = changedEventHandler;
      changedEventHandler = Interlocked.CompareExchange<PropertyChangedEventHandler>(ref this.PropertyChanged, comparand + value, comparand);
    }
    while (changedEventHandler != comparand);
  }
  remove
  {
    PropertyChangedEventHandler changedEventHandler = this.PropertyChanged;
    PropertyChangedEventHandler comparand;
    do
    {
      comparand = changedEventHandler;
      changedEventHandler = Interlocked.CompareExchange<PropertyChangedEventHandler>(ref this.PropertyChanged, comparand - value, comparand);
    }
    while (changedEventHandler != comparand);
  }
}

我无法理解的是为什么PropertyChanged有两个（公共和私有）实现，但只有一个（受保护的）CollectonChanged？那个代表算术在do-while循环中的目的是什么？为什么不像PropertyChanged的私有实现那样只是“this.Property + = value”？

Answer 1

避免使用反编译器查看框架代码，使用可以从Reference Source获取的实际源代码。此类的源位于Source.NET 4.5 \ 4.6.0.0 \ net \ fx \ src \ CompMod \ System \ Collections \ ObjectModel \ ObservableCollection.cs目录中。然后，您将看到CollectionChanged事件真正的样子：

    //------------------------------------------------------
    /// <summary>
    /// Occurs when the collection changes, either by adding or removing an item.
    /// </summary>
    /// <remarks>
    /// see <seealso cref="INotifyCollectionChanged"/>
    /// </remarks>
    [field:NonSerializedAttribute()]
    public virtual event NotifyCollectionChangedEventHandler CollectionChanged;

就是这样，没有更多的代码。

反编译器不够智能，无法识别C＃编译器为实现事件而生成的自动生成的代码，因此不知道如何将其折叠回真正的 C＃代码事件。至少部分原因是这种缺陷在.NET 4.0中发生了变化。 do-while循环是新的，它可以确保事件订阅线程安全。较旧的方法使用[MethodImpl（MethodImplOptions.Synchronized）]属性使其成为线程安全的。它有一个缺陷，它会锁定这个，这种做法可能导致无意的死锁。

是的，有两个PropertyChanged事件。一个是INotifyPropertyChanged的显式实现，它只使用另一个。这样做是为了确保事件不是公共的，就像您继承接口时通常会发生的那样，它被重新声明为 protected 。

ObservableCollection事件中的神秘代表算术实现

1 个答案: