问题描述
我正在尝试编写一个通用函数,该函数给出了对控件/组件的引用以及在其类上声明的事件的名称,它应该能够(通过 Reflection )进行检索当前为指定事件名称注册的所有事件处理程序.
I'm trying to write a universal function that, given a reference to a control/component and the name of an event declared on its class, it should be able to retrieve (through Reflection) all the event-handlers currently registered for the specified event name.
我遇到的第一个也是主要的问题(已解决,因此您可以忽略此段)是,我在StackOverflow中发现的所有解决方案(大多是用C#编写)都受到作者只能寻找的意义的限制.System.Windows.Forms.Control 类中的事件字段声明,因此,例如,当尝试检索 System.Windows.Forms.ToolStripMenuItem的事件处理程序时,该事件将失败..MouseEnter
事件(因为事件字段是在 System.Windows.Forms.ToolStripItem
类中声明的),并且也不考虑 System的事件字段命名.Windows.Forms.Form
类,具有下划线.因此,我涵盖了所有这些内容,目前,我的解决方案适用于(或我认为它适用于)从 System.ComponentModel.Component
继承的任何类.
The first and main problem I had (which is solved, so you can ignore this paragraph), is that all the solutions (mostly written in C#) that I found in StackOverflow are limited in the meaning that the authors only look for the event-field declaration in the System.Windows.Forms.Control
class, and for that reason will fail for example when trying to retrieve the event-handlers of System.Windows.Forms.ToolStripMenuItem.MouseEnter
event (since the event-field is declared in System.Windows.Forms.ToolStripItem
class), and also does not take into account event-fields naming of System.Windows.Forms.Form
class, which have a underscore.So I covered all this, and currently my solution works (or I think it works) for any class that inherits from System.ComponentModel.Component
.
我现在遇到的唯一问题是当我声明自定义类型(继承自 Control / UserControl / Component / Form 等类),然后将该类型传递给函数.在这种情况下,我会得到一个空引用异常.不知道我在这里做错了什么...
The only problem I'm having now is when I declare a custom type (that inherits from Control / UserControl / Component / Form etc. class) and I pass that type to my function. In this circumstance I get a null-reference exception. Not sure what I'm doing wrong here...
Public Shared Function GetEventHandlers(component As IComponent, eventName As String) As IReadOnlyCollection(Of [Delegate])
Dim componentType As Type
Dim declaringType As Type ' The type on which the event is declared.
Dim eventInfo As EventInfo
Dim eventField As FieldInfo = Nothing
Dim eventFieldValue As Object
Dim eventsProp As PropertyInfo
Dim eventsPropValue As EventHandlerList
Dim eventDelegate As [Delegate]
Dim invocationList As [Delegate]()
' Possible namings for an event field.
Dim eventFieldNames As String() =
{
$"Event{eventName}", ' Fields declared in 'System.Windows.Forms.Control' class.
$"EVENT_{eventName.ToUpper()}", ' Fields declared in 'System.Windows.Forms.Form' class.
$"{eventName}Event" ' Fields auto-generated.
}
Const bindingFlagsEventInfo As BindingFlags =
BindingFlags.ExactBinding Or
BindingFlags.Instance Or
BindingFlags.NonPublic Or
BindingFlags.Public Or
BindingFlags.Static
Const bindingFlagsEventField As BindingFlags =
BindingFlags.DeclaredOnly Or
BindingFlags.ExactBinding Or
BindingFlags.IgnoreCase Or
BindingFlags.Instance Or
BindingFlags.NonPublic Or
BindingFlags.Static
Const bindingFlagsEventsProp As BindingFlags =
BindingFlags.DeclaredOnly Or
BindingFlags.ExactBinding Or
BindingFlags.Instance Or
BindingFlags.NonPublic
Const bindingFlagsEventsPropValue As BindingFlags =
BindingFlags.Default
componentType = component.GetType()
eventInfo = componentType.GetEvent(eventName, bindingFlagsEventInfo)
If (eventInfo Is Nothing) Then
Throw New ArgumentException($"Event with name '{eventName}' not found in type '{componentType.FullName}'.", NameOf(eventName))
End If
declaringType = eventInfo.DeclaringType
For Each name As String In eventFieldNames
eventField = declaringType.GetField(name, bindingFlagsEventField)
If (eventField IsNot Nothing) Then
Exit For
End If
Next name
If (eventField Is Nothing) Then
Throw New ArgumentException($"Field with name 'Event{eventName}', 'EVENT_{eventName.ToUpper()}' or '{eventName}Event' not found in type '{declaringType.FullName}'.", NameOf(eventName))
End If
#If DEBUG Then
Debug.WriteLine($"Field with name '{eventField.Name}' found in type '{declaringType.FullName}'")
#End If
eventFieldValue = eventField.GetValue(component)
eventsProp = GetType(Component).GetProperty("Events", bindingFlagsEventsProp, Type.DefaultBinder, GetType(EventHandlerList), Type.EmptyTypes, Nothing)
eventsPropValue = DirectCast(eventsProp.GetValue(component, bindingFlagsEventsPropValue, Type.DefaultBinder, Nothing, CultureInfo.InvariantCulture), EventHandlerList)
eventDelegate = eventsPropValue.Item(eventFieldValue)
invocationList = eventDelegate.GetInvocationList()
If (invocationList Is Nothing) Then ' There is no event-handler registered for the specified event.
Return Enumerable.Empty(Of [Delegate]).ToList()
End If
Return invocationList
End Function
此行发生异常:
invocationList = eventDelegate.GetInvocationList()
因为 eventDelegate
为空.
要测试该异常,可以以此类为例:
To test the exception, you can take this class as example:
Public Class TestUserControl : Inherits UserControl
Event TestEvent As EventHandler(Of EventArgs)
Overridable Sub OnTestEvent(e As EventArgs)
If (Me.TestEventEvent IsNot Nothing) Then
RaiseEvent TestEvent(Me, e)
End If
End Sub
End Class
还有这样的用法示例:
Dim ctrl As New TestUserControl()
AddHandler ctrl.TestEvent, Sub()
Debug.WriteLine("Hello World!")
End Sub
Dim handlers As IReadOnlyCollection(Of [Delegate]) =
GetEventHandlers(ctrl, NameOf(TestUserControl.TestEvent))
For Each handler As [Delegate] In handlers
Console.WriteLine($"Method Name: {handler.Method.Name}")
Next
不确定是否是与绑定标志有关的问题,或者事件字段的命名...但是在与任何内置控件/组件尝试相同时,我没有此空引用对象问题暴露事件的类,而不是该 TestUserControl
类.
Not sure if maybe it is an issue related to the binding flags, or maybe the event field naming... but I don't have this null-reference object issue when trying the same with any built-in control/component class that expose events, instead of that TestUserControl
class.
我做错了什么?如何解决?请注意,此功能仍应是通用的.
What I'm doing wrong?, and how do I fix it?. Please note that this function should still be universal.
推荐答案
感谢@ Hans Passant 在主要问题的评论中提出的建议,该方法可以正常工作:
Thanks to what @Hans Passant suggested in his commentary in the main question, this is working as expected:
Public Function GetEventHandlers(component As IComponent, eventName As String) As IReadOnlyCollection(Of [Delegate])
Dim componentType As Type = component.GetType()
' Find event declaration in the source type.
Dim eventInfo As EventInfo = componentType.GetEvent(eventName, BindingFlags.ExactBinding Or BindingFlags.Instance Or BindingFlags.NonPublic Or BindingFlags.Public Or BindingFlags.Static)
If (eventInfo Is Nothing) Then
Throw New ArgumentException($"Event with name '{eventName}' not found in type '{componentType.FullName}'.", NameOf(eventName))
End If
' The type on which the event is declared.
Dim declaringType As Type = eventInfo.DeclaringType
' Find event-field declaration in the declaring type.
Dim eventField As FieldInfo = Nothing
' Possible namings for an event field.
Dim eventFieldNames As String() = {
$"Event{eventName}", ' Fields declared in 'System.Windows.Forms.Control' class.
$"EVENT_{eventName.ToUpper()}", ' Fields declared in 'System.Windows.Forms.Form' class.
$"{eventName}Event" ' Fields (auto-generated) declared in other classes.
}
For Each name As String In eventFieldNames
eventField = declaringType.GetField(name, BindingFlags.DeclaredOnly Or BindingFlags.ExactBinding Or BindingFlags.IgnoreCase Or BindingFlags.Instance Or BindingFlags.NonPublic Or BindingFlags.Static)
If (eventField IsNot Nothing) Then
Exit For
End If
Next name
If (eventField Is Nothing) Then
Throw New ArgumentException($"Field with name '{String.Join("' or '", eventFieldNames)}' not found in declaring type '{declaringType.FullName}'.", NameOf(eventName))
End If
#If DEBUG Then
Debug.WriteLine($"Field with name '{eventField.Name}' found in declaring type '{declaringType.FullName}'")
#End If
Dim eventFieldValue As object = eventField.GetValue(component)
If TypeOf eventFieldValue Is MulticastDelegate
' See @Hans Passant comment:
' https://stackoverflow.com/questions/56763972/get-all-the-event-handlers-of-a-event-declared-in-a-custom-user-control?noredirect=1#comment100177090_56763972
Return DirectCast(eventFieldValue, MulticastDelegate).GetInvocationList()
End If
Dim eventsProp As PropertyInfo = GetType(Component).GetProperty("Events", BindingFlags.DeclaredOnly Or BindingFlags.ExactBinding Or BindingFlags.Instance Or BindingFlags.NonPublic, Type.DefaultBinder, GetType(EventHandlerList), Type.EmptyTypes, Nothing)
Dim eventsPropValue As EventHandlerList = DirectCast(eventsProp.GetValue(component, BindingFlags.Default, Type.DefaultBinder, Nothing, CultureInfo.InvariantCulture), EventHandlerList)
Dim eventDelegate As [Delegate] = eventsPropValue.Item(eventFieldValue)
Dim invocationList As [Delegate]() = eventDelegate?.GetInvocationList()
If (invocationList Is Nothing) Then ' There is no event-handler registered for the specified event.
Return Enumerable.Empty(Of [Delegate]).ToList()
End If
Return invocationList
End Function
我们还可以为 EventInfo 类型定义下一个方法扩展,以充当 EventInfo.RemoveEventHandler(Object,Delegate):
Also we can define the next method extension for EventInfo type to act as a method overload for EventInfo.RemoveEventHandler(Object, Delegate):
''' ----------------------------------------------------------------------------------------------------
''' <summary>
''' Removes an event handler from an event source.
''' </summary>
''' ----------------------------------------------------------------------------------------------------
''' <example> This is a code example.
''' <code>
''' Public Class Form1
'''
''' Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Shown
''' Dim target As Form = Me
''' Dim eventInfo As EventInfo = target.GetType().GetEvent(NameOf(Form.Click))
''' eventInfo.RemoveEventHandler(target, NameOf(Me.Form1_Click))
''' End Sub
'''
''' Private Sub Form1_Click(sender As Object, e As EventArgs) Handles MyBase.Click
''' MsgBox(MethodBase.GetCurrentMethod().Name)
''' End Sub
'''
''' End Class
''' </code>
''' </example>
''' ----------------------------------------------------------------------------------------------------
''' <param name="eventInfo">
''' The event information.
''' </param>
'''
''' <param name="target">
''' The event source.
''' </param>
'''
''' <param name="handlerName">
''' The name of the delegate to be disassociated from the events raised by <paramref name="target"/>.
''' <para></para>
''' Note that the name is case-sensitive.
''' </param>
''' ----------------------------------------------------------------------------------------------------
<Extension>
Public Sub RemoveEventHandler(eventInfo As EventInfo, target As IComponent, handlerName As String)
If String.IsNullOrWhiteSpace(handlerName)
Throw New ArgumentNullException(NameOf(handlerName))
End If
For each handler As [Delegate] in GetEventHandlers(target, eventInfo.Name)
If handler.Method.Name.Equals(handlerName, StringComparison.Ordinal)
eventInfo.RemoveEventHandler(target, handler)
Exit Sub
End If
Next handler
Throw New ArgumentException($"No delegate was found with the specified name: '{handlerName}'", NameOf(handlerName))
End Sub
这篇关于获取在自定义用户控件中声明的事件的所有事件处理程序的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!