回答我自己的问题:可以恢复已被上游吞噬的拖放事件。

首先，我使用了IOleDropTarget接口，如下所示；

 <ComImport, Guid("00000122-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>
Interface IOleDropTarget
    <PreserveSig>
    Function OleDragEnter(
    <[In], MarshalAs(UnmanagedType.[Interface])> ByVal pDataObj As Object,
    <[In], MarshalAs(UnmanagedType.U4)> ByVal grfKeyState As Integer,
    <[In], MarshalAs(UnmanagedType.U8)> ByVal pt As Long,
    <[In], Out> ByRef pdwEffect As Integer) As Integer
    <PreserveSig>
    Function OleDragOver(
    <[In], MarshalAs(UnmanagedType.U4)> ByVal grfKeyState As Integer,
    <[In], MarshalAs(UnmanagedType.U8)> ByVal pt As Long,
    <[In], Out> ByRef pdwEffect As Integer) As Integer
    <PreserveSig>
    Function OleDragLeave() As Integer
    <PreserveSig>
    Function OleDrop(
    <[In], MarshalAs(UnmanagedType.[Interface])> ByVal pDataObj As Object,
    <[In], MarshalAs(UnmanagedType.U4)> ByVal grfKeyState As Integer,
    <[In], MarshalAs(UnmanagedType.U8)> ByVal pt As Long,
    <[In], Out> ByRef pdwEffect As Integer) As Integer
End Interface

接下来，当我们在界面上时，让我们创建当文件被拖动到这个界面时windows资源管理器所显示的漂亮图标。

Imports IDataObject_Com = System.Runtime.InteropServices.ComTypes.IDataObject
Imports System.Windows.Interop
Imports System.Runtime.InteropServices
Namespace Browser
<StructLayout(LayoutKind.Sequential)>
Public Structure Win32Point
    Public x As Integer
    Public y As Integer
End Structure
<ComImport>
<Guid("4657278A-411B-11d2-839A-00C04FD918D0")>
Public Class DragDropHelper
End Class
<ComVisible(True)>
<ComImport>
<Guid("4657278B-411B-11D2-839A-00C04FD918D0")>
<InterfaceType(ComInterfaceType.InterfaceIsIUnknown)>
Interface IDropTargetHelper
    Sub DragEnter(
 <[In]> ByVal hwndTarget As IntPtr,
 <[In], MarshalAs(UnmanagedType.[Interface])> ByVal dataObject As IDataObject_Com,
 <[In]> ByRef pt As Win32Point,
 <[In]> ByVal effect As Integer)
    Sub DragLeave()
    Sub DragOver(
 <[In]> ByRef pt As Win32Point,
 <[In]> ByVal effect As Integer)
    Sub Drop(
 <[In], MarshalAs(UnmanagedType.[Interface])> ByVal dataObject As IDataObject_Com,
 <[In]> ByRef pt As Win32Point,
 <[In]> ByVal effect As Integer)
    Sub Show(
 <[In]> ByVal show As Boolean)
End Interface

您需要实现IOleDropTaget接口，然后该接口将提供DragEnter、Over、Leave和Drop事件。

要挂接这些事件，您不需要了解关于messageloop或wndproc消息的任何信息，如上所述。你需要知道的是，其中一个类名称为"Chrome_WidgetWin_0“的Chromium窗口是为拖放注册的，在获得事件之前，必须首先撤销这个窗口。

CefSharp示例展示了如何深入到Chromium窗口，但这通常是为了获得另一个窗口类。在本例中，我使用以下代码(注意，我撤销了在回调函数中找到的所有窗口，但似乎只注册了Chrome_WidgetWin_0。

Const Chrome_WidgetWin As String = "Chrome_WidgetWin_0"
    Private Function TryFindHandl(ByVal browserHandle As IntPtr, <Out> ByRef chromeWidgetHostHandle As IntPtr) As Boolean
        Dim cbXL As New NativeMethodsEx.EnumChildCallback(AddressOf EnumChildProc_Browser)
        NativeMethodsEx.EnumChildWindows(browserHandle, cbXL, chromeWidgetHostHandle)
        Return chromeWidgetHostHandle <> IntPtr.Zero
    End Function
    Private Shared Function EnumChildProc_Browser(ByVal hwndChild As Integer, ByRef lParam As Integer) As Boolean
        Dim buf As New StringBuilder(128)
        NativeMethodsEx.GetClassName(hwndChild, buf, 128)
        Dim ret = NativeMethodsEx.RevokeDragDrop(hwndChild)
        If ret = NativeMethodsEx.DRAGDROP_E_NOTREGISTERED Then
            Debug.Print("")
        End If
        If buf.ToString = Chrome_WidgetWin Then
            lParam = hwndChild
            Return False
        End If
        Return True
    End Function

一旦你有了这个句柄，并且你已经撤销了它作为拖放目标，那么你就可以调用RegisterDragDrop，传入句柄和你的IOleDropTarget类。

我的一些WinAPI签名如下所示

 Friend Const DRAGDROP_E_NOTREGISTERED = &H80040100
Friend Const DRAGDROP_E_INVALIDHWND = &H80040102
Friend Const DRAGDROP_E_ALREADYREGISTERED = &H80040101
Friend Const E_OUTOFMEMORY = &H8007000E
Friend Const S_OK = 0
<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
Friend Shared Function GetClassName(ByVal hWnd As System.IntPtr, ByVal lpClassname As System.Text.StringBuilder, ByVal nMaxCount As Integer) As Integer
End Function
Friend Delegate Function EnumChildCallback(ByVal hwnd As Integer, ByRef lParam As Integer) As Boolean
<DllImport("User32.dll")>
Friend Shared Function EnumChildWindows(ByVal hWndParent As Integer, ByVal lpEnumFunc As EnumChildCallback, ByRef lParam As Integer) As Boolean
End Function
<DllImport("ole32.dll")>
Friend Shared Function RegisterDragDrop(ByVal hwnd As IntPtr, DropTarget As Browser.IOleDropTarget) As IntPtr
End Function
<DllImport("ole32.dll")>
Friend Shared Function RevokeDragDrop(ByVal hwnd As IntPtr) As IntPtr
End Function

该事件的示例以及如何使用IDropTargetHelper如下所示

Public Function OleDragEnter(<[In]> <MarshalAs(UnmanagedType.Interface)> pDataObj As Object, <[In]> <MarshalAs(UnmanagedType.U4)> grfKeyState As Integer, <[In]> <MarshalAs(UnmanagedType.U8)> pt As Long, <[In]> <Out> ByRef pdwEffect As Integer) As Integer Implements IOleDropTarget.OleDragEnter
        Dim winPT As Win32Point
        winPT.x = CInt(pt And &H7FFFFFFF)
        winPT.y = CInt((pt >> 32) And &H7FFFFFFF)
        Dim eff As DragDropEffects = DragDropEffects.None
        'this is my event I am sending back to the browser class to deal with.
        RaiseEvent DBDragEnter(eff, New Point(winPT.x, winPT.y))