好问题。
一般情况下，您不能乱用密封类的事件处理程序。通常，您可以覆盖A 的假设OnPropertyChanged 并根据某些标志引发或不引发事件。或者，您可以提供一个不引发事件的 setter 方法，正如@Vadim 所建议的那样。但是，如果 A 是密封的，您最好的选择是向列表器添加标志，就像您所做的那样。这将使您能够识别由B 引发的PropertyChanged 事件，但您将无法为其他侦听器抑制该事件。
现在，既然您提供了上下文... 有一种在 WPF 中执行此操作的方法。所有需要做的是B 的TextBox.TextChanged 处理程序需要设置e.Handled = _dontDoThis。如果B 的一个被添加为第一个，这将抑制所有其他听众的通知。如何确保发生这种情况？反思！
UIElement 仅公开AddHandler 和RemoveHandler 方法，没有InsertHandler 允许手动指定处理程序的优先级。但是，快速查看 .NET 源代码（下载 the whole thing 或 query what you need）会发现 AddHandler 将参数转发给内部方法 EventHandlersStore.AddRoutedEventHandler，它执行以下操作：
// Create a new RoutedEventHandler
RoutedEventHandlerInfo routedEventHandlerInfo =
new RoutedEventHandlerInfo(handler, handledEventsToo);
// Get the entry corresponding to the given RoutedEvent
FrugalObjectList<RoutedEventHandlerInfo> handlers = (FrugalObjectList<RoutedEventHandlerInfo>)this[routedEvent];
if (handlers == null)
{
_entries[routedEvent.GlobalIndex] = handlers = new FrugalObjectList<RoutedEventHandlerInfo>(1);
}
// Add the RoutedEventHandlerInfo to the list
handlers.Add(routedEventHandlerInfo);
所有这些东西都是内部的，但可以使用反射重新创建：
public static class UIElementExtensions
{
public static void InsertEventHandler(this UIElement element, int index, RoutedEvent routedEvent, Delegate handler)
{
// get EventHandlerStore
var prop = typeof(UIElement).GetProperty("EventHandlersStore", BindingFlags.NonPublic | BindingFlags.Instance);
var eventHandlerStoreType = prop.PropertyType;
var eventHandlerStore = prop.GetValue(element, new object[0]);
// get indexing operator
PropertyInfo indexingProperty = eventHandlerStoreType.GetProperties(BindingFlags.NonPublic | BindingFlags.Instance)
.Single(x => x.Name == "Item" && x.GetIndexParameters().Length == 1 && x.GetIndexParameters()[0].ParameterType == typeof(RoutedEvent));
object handlers = indexingProperty.GetValue(eventHandlerStore, new object[] { routedEvent });
if (handlers == null)
{
// just add the handler as there are none at the moment so it is going to be the first one
if (index != 0)
{
throw new ArgumentOutOfRangeException("index");
}
element.AddHandler(routedEvent, handler);
}
else
{
// create routed event handler info
var constructor = typeof(RoutedEventHandlerInfo).GetConstructors(BindingFlags.NonPublic | BindingFlags.Instance).Single();
var handlerInfo = constructor.Invoke(new object[] { handler, false });
var insertMethod = handlers.GetType().GetMethod("Insert");
insertMethod.Invoke(handlers, new object[] { index, handlerInfo });
}
}
}
现在调用 InsertEventHandler(0, textBox, TextBox.TextChangedEvent, new TextChangedEventHandler(textBox_TextChanged)) 将确保您的处理程序将是列表中的第一个，使您能够禁止其他侦听器的通知！
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
var textBox = new TextBox();
textBox.TextChanged += (o, e) => Console.WriteLine("External handler");
var b = new B(textBox);
textBox.Text = "foo";
b.MakeProblem();
}
}
class B
{
private TextBox _a;
bool _dontDoThis;
public B(TextBox a)
{
_a = a;
a.InsertEventHandler(0, TextBox.TextChangedEvent, new TextChangedEventHandler(Handler));
}
void Handler(object sender, TextChangedEventArgs e)
{
Console.WriteLine("B.Handler");
e.Handled = _dontDoThis;
if (_dontDoThis)
{
e.Handled = true;
return;
}
// do this!
}
public void MakeProblem()
{
try
{
_dontDoThis = true;
_a.Text = "make a problem";
}
finally
{
_dontDoThis = false;
}
}
}
输出：
B.Handler
External handler
B.Handler