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

Question

我正在尝试编写一个通用函数，给定对控件/组件的引用以及在其类上声明的事件的名称，它应该能够检索(通过反射)当前为指定事件名称注册的所有事件处理程序。

我遇到的第一个也是主要问题(已解决，因此您可以忽略此段)是我在 StackOverflow 中找到的所有解决方案(主要是用 C# 编写的)都受到限制，因为作者只在System.Windows.Forms.Control类中查找事件字段声明，因此例如在尝试检索System.Windows.Forms.ToolStripMenuItem.MouseEnter事件的事件处理程序时会失败(因为事件字段是在System.Windows.Forms.ToolStripItem类)，并且也不考虑System.Windows.Forms.Form类的事件字段命名，这些字段具有下划线。 所以我涵盖了所有这些，目前我的解决方案适用于(或者我认为它适用于从System.ComponentModel.Component继承的任何类。

我现在遇到的唯一问题是当我声明一个自定义类型(继承自控件/用户控件/组件/窗体等.class)并将该类型传递给我的函数时。在这种情况下，我得到一个空引用异常。不知道我在这里做错了什么...

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

异常发生在以下行：

invocationList = eventDelegate.GetInvocationList()

因为eventDelegate为空。

---

要测试异常，可以以此类为例：

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

下面是一个这样的用法示例：

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

不确定这是否与绑定标志有关，或者可能是事件字段命名的问题......但是当尝试使用任何公开事件的内置控件/组件类而不是该TestUserControl类进行相同的操作时，我没有这个 null 引用对象问题。

我做错了什么？，我该如何解决它？请注意，此功能仍应是通用的。

Answer 1

感谢@Hans Passant在主要问题的评论中提出的建议，这正在按预期工作：

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

此外，我们可以为EventInfo类型定义下一个方法扩展，以充当 EventInfo.RemoveEventHandler(Object， Delegate) 的方法重载：

''' ----------------------------------------------------------------------------------------------------
''' <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