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[英]How do I pass an argument to an event handler when the event handler is a delegate and doesn't include that argument?
[英]How do I cast an event handler delegate to one with a different signature
我編寫的代碼實際上是WPF行為,用於從網格控件中獲取選定的項(眾所周知,SelectedItems不是可綁定的屬性)。 我實際上正在使用Telerik RadGridView,但是我希望該行為對於帶有SelectionChanged事件的任何事物都是通用的。 但是,不同的控件對SelectionChanged事件處理程序具有不同的簽名(RadGridView使用Telerik.Windows.Controls.SelectionChangeEventArgs,而標准GridView使用System.Windows.Controls.SelectionChangedEventArgs)。 我們可以確定的一件事是,事件args將從EventArgs派生(實際上,我們可以確保它將從RoutedEventArgs派生)。
但是,盡管我可以編寫一個將RoutedEventArgs作為其第二個參數的通用事件處理程序,並且可以使用反射來獲取SelectionChangedEvent的EventInfo,但如果不使用事件的精確簽名,就無法將處理程序掛接到事件上處理程序-在這種情況下為RadGridView處理程序。
這是我的代碼。 我已經包含了所有內容,但重要的是SelectItemPropertyChanged,它是DependencyObject PropertyChangedCallback,它試圖將事件處理程序SelectionChangedHandler連接到SelectionChangedEvent。 (SelectionChangedHandler中的代碼與該問題無關,但是我將其保留了下來,因此很清楚我在做什么)。
public static class SelectedItemsChangedBehaviour{
public static readonly DependencyProperty SelectItemsProperty =
DependencyProperty.RegisterAttached("SelectItems", typeof(bool), typeof(SelectedItemsChangedBehaviour),
new FrameworkPropertyMetadata(false, new PropertyChangedCallback(SelectItemPropertyChanged)));
public static void SetSelectItems(DependencyObject dependencyObject, bool selectItems)
{
dependencyObject.SetValue(SelectItemsProperty, selectItems);
}
public static bool GetSelectItems(DependencyObject dependencyObject)
{
return (bool)dependencyObject.GetValue(SelectItemsProperty);
}
private static void SelectItemPropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
{
// No common base for all classes with SelectionChanged event so use reflection
EventInfo selectionChangedEventInfo = dependencyObject.GetType().GetEvent("SelectionChanged");
if (selectionChangedEventInfo == null)
{
throw new ArgumentException("Must have a SelectionChanged event.");
}
if ((bool)dependencyPropertyChangedEventArgs.OldValue)
{
// This is what I want to do, but it throws because the handler signature is wrong
selectionChangedEventInfo.RemoveEventHandler(dependencyObject, (RoutedEventHandler)SelectionChangedHandler);
// This works fine because it is of the right type for the RadGridView but is not general
//selectionChangedEventInfo.RemoveEventHandler(dependencyObject, (EventHandler<Telerik.Windows.Controls.SelectionChangeEventArgs>)SelectionChangedHandler);
}
if ((bool)dependencyPropertyChangedEventArgs.NewValue)
{
// This is what I want to do, but it throws because the handler signature is wrong
selectionChangedEventInfo.AddEventHandler(dependencyObject, (RoutedEventHandler)SelectionChangedHandler);
// This works fine because it is of the right type for the RadGridView but is not general
//selectionChangedEventInfo.AddEventHandler(dependencyObject, (EventHandler<Telerik.Windows.Controls.SelectionChangeEventArgs>)SelectionChangedHandler);
}
}
private static void SelectionChangedHandler(object sender, RoutedEventArgs eventArgs)
{
// No common base for all classes with AddedItems/RemovedItems (eg. System.Windows.Controls.SelectionChangedEventArgs / Telerik.Windows.Controls.SelectionChangeEventArgs
PropertyInfo addedItemsInfo = eventArgs.GetType().GetProperty("AddedItems");
PropertyInfo removedItemsInfo = eventArgs.GetType().GetProperty("RemovedItems");
if (addedItemsInfo == null || removedItemsInfo == null)
{
throw new ArgumentException("Must have AddedItems and RemovedItems");
}
foreach (object item in (IEnumerable)addedItemsInfo.GetValue(eventArgs, null))
{
((ISelectable)item).IsSelected = true;
}
foreach (object item in (IEnumerable)removedItemsInfo.GetValue(eventArgs, null))
{
((ISelectable)item).IsSelected = false;
}
}
我嘗試了各種方法來使用反射為處理程序獲取正確的簽名,從而為正確的類型創建一個委托,但我無法使其正常工作-AddEventHandler(和RemoveEventHandler)拋出InvalidArgumentException,完整的堆棧跟蹤如下:
{“類型'System.Windows.RoutedEventHandler'的對象不能轉換為類型'System.EventHandler`1 [Telerik.Windows.Controls.SelectionChangeEventArgs]'。”}
在System.RuntimeType.TryChangeType處(對象值,活頁夾活頁夾,CultureInfo文化,布爾布爾NeedSpecialCast)
有人可以建議嗎?
調用AddEventHandler
時,需要將委托轉換為事件的EventHandlerType
。 這是一個示例應用程序:
using System;
using System.Reflection;
using System.Threading;
namespace App
{
class Program
{
public event EventHandler<ThreadExceptionEventArgs> E;
static void Main ()
{
new Program().Run();
}
private void Run ()
{
EventInfo e = typeof(Program).GetEvent("E");
EventHandler untypedHandler = OnE;
Delegate typedHandler = Delegate.CreateDelegate(e.EventHandlerType,
untypedHandler.Target, untypedHandler.Method);
e.AddEventHandler(this, typedHandler);
E(this, new ThreadExceptionEventArgs(new Exception("Hello world!")));
}
private void OnE (object sender, EventArgs args)
{
Console.WriteLine(((ThreadExceptionEventArgs)args).Exception.Message);
}
}
}
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