获取自定义用户控件中声明的事件的所有事件处理程序

我正在尝试编写一个通用函数，给出对控件/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，因此您可以忽略本段）是我在 StackOverflow 中找到的所有解决方案（主要是用 C# 编写的）都仅限于作者仅查找事件的含义 -中的字段声明System.Windows.Forms.Control类，因此会失败，例如在尝试检索事件处理程序时System.Windows.Forms.ToolStripMenuItem.MouseEnter事件（因为事件字段是在System.Windows.Forms.ToolStripItem类），并且也不考虑事件字段命名System.Windows.Forms.Form类，其中有一个下划线。 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）继承自的任何类System.ComponentModel.Component.

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Control / 用户控制 / 成分 / Form等），然后我将该类型传递给我的函数。在这种情况下，我遇到了空引用异常。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

异常发生在这一行：

invocationList = eventDelegate.GetInvocationList()

because eventDelegate一片空白。

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

和这样的示例用法：

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

不确定这是否是与绑定标志相关的问题，或者可能是与事件字段命名相关的问题...但在尝试与公开的任何内置控件/组件类相同时，我没有这个空引用对象问题事件，而不是那个TestUserControl class.

我做错了什么？以及如何解决？请注意，此功能仍然应该是通用的。

感谢什么@汉斯·帕桑特在主要问题的评论中建议，这按预期工作：

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

我们还可以定义下一个方法扩展事件信息充当方法重载的类型EventInfo.RemoveEventHandler（对象，委托） https://learn.microsoft.com/en-us/dotnet/api/system.reflection.eventinfo.removeeventhandler?view=netframework-4.8:

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