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到現在爲止還沒有任何關於接口的知識，但我實際上正在努力實施this question的解決方案。從接口指針獲取接口/對象的正確方法？
我想我正確地獲取指向IShellFolder接口的指針，但我似乎無法實際使用該指針。如何獲得一個可用的接口，當有指針？
下面的代碼顯示了我在做什麼以及我懷疑存在哪些問題。
首先，我接口delarations：
' I have wrapped the interfaces in their own namespace, "BinarusShell".
' I have declared every interface's functions/subs in the same order as they are in the .idl files of the Windows Platform SDK 7.1.
' Every function/sub has the <PreserveSig()> attribute so that I can see the native return values when debugging.
' I'd like to keep the question as lean as possible, so I am showing only a few functions per interface.
' Of course, I have implemented all of them (as they are in the respective .idl file).
Namespace BinarusShell
<ComImport()> _
<InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _
<Guid("000214FA-0000-0000-C000-000000000046")> _
Public Interface IExtractIcon
' The above GUID is the GUID for the UNICODE version of the interface, i.e. IExtractIconW.
' Therefore, the UNICODE variant of strings is used in the parameters of the following functions.
' See also the include files of the Windows SDK where this is done exactly that way as well.
<PreserveSig()> Function GetIconLocation(ByVal uFlags As UInteger,
<MarshalAs(UnmanagedType.LPWStr, SizeParamIndex:=2)> ByVal pszIconFile As String,
ByVal cchmax As UInteger,
ByRef piIndex As Integer,
ByRef pwFlags As UInteger) As Integer
' ...
' Here comes the second function (IExtractIcon has only two) ...
' ...
End Interface
<ComImport()> _
<InterfaceType(ComInterfaceType.InterfaceIsIUnknown)> _
<Guid("000214E6-0000-0000-C000-000000000046")> _
Public Interface IShellFolder
<PreserveSig()> Function GetUIObjectOf(<[In]()> ByVal hwndOwner As IntPtr,
<[In]()> ByVal cidl As UInteger,
<[In]()> <MarshalAs(UnmanagedType.LPArray, SizeParamIndex:=1)> ByVal apidl() As IntPtr,
<[In]()> <MarshalAs(UnmanagedType.LPStruct)> ByVal riid As Guid,
<[In](), Out()> ByRef rgfReserved As UInteger,
<Out()> ByRef ppv As IntPtr) As Integer
' ...
' Here come the other functions ...
' ...
End Interface
End Namespace
其次，我的Windows API聲明，並 三，功能我想實現（這是我在哪裏卡住）：
' The Windows API declarations are in class Win32Api.
' This class is not wrapped in a certain namespace.
' To keep this question as lean as possible, I am leaving out my declarations of constants or GUIDs.
' I have taken all of them literally from the Windows Platform SDK 7.1.
Public Class Win32Api
<DllImport("shell32.dll", CharSet:=CharSet.Auto)> _
Private Shared Function SHBindToParent(<[In]()> ByVal pidl As IntPtr,
<[In]()> <MarshalAs(UnmanagedType.LPStruct)> ByVal riid As Guid,
<Out()> ByRef ppv As IntPtr,
<Out()> ByRef ppidlLast As IntPtr) As Integer
End Function
<DllImport("shell32.dll", CharSet:=CharSet.Auto)> _
Private Shared Function SHGetKnownFolderIDList(<[In]()> <MarshalAs(UnmanagedType.LPStruct)> ByVal rfid As Guid,
<[In]()> ByVal dwFlags As UInteger,
<[In]()> ByVal hToken As IntPtr,
<Out()> ByRef ppidl As IntPtr) As Integer
End Function
' -----------------------------------------------------
' The function I would like to implement is in class Win32Api as well.
' The function at this stage does nothing which is useful for the outer world;
' It is just meant for code which I am stepping through with the debugger.
' The comments below show the result of every step and my understanding about what has happened.
Public Shared Function IconTest() As Boolean
Dim ui_ReturnFlags As UInteger
Dim i_Result As Integer
Dim intptr_CurrentPidlAbsolute As IntPtr,
intptr_ParentIShellFolder As IntPtr,
intptr_CurrentIExtractIcon As IntPtr
Dim arintptr_RelPidList(0) As IntPtr
Dim ishellfolder_Parent As BinarusShell.IShellFolder
Dim iextracticon_Extractor As BinarusShell.IExtractIcon
i_Result = SHGetKnownFolderIDList(FOLDERID_System, 0, 0, intptr_CurrentPidlAbsolute)
' The above seems to work. i_Result is 0 which means success, and
' intptr_CurrentPidlAbsolute has a reasonable value now.
i_Result = SHBindToParent(intptr_CurrentPidlAbsolute, IID_IShellFolder, intptr_ParentIShellFolder, arintptr_RelPidList(0))
' The above seems to work. i_Result is 0 which means success, and
' intptr_ParentIShellFolder and arintptr_RelPidList(0) have reasonable values now.
' Now the PROBLEMS begin. I have no clue how to get an interface/object
' from intptr_ParentIShellFolder which I can use from within VB.net.
' Therefore, I just have tried the following (but believing that it is wrong):
ishellfolder_Parent = DirectCast(Marshal.GetObjectForIUnknown(intptr_ParentIShellFolder), BinarusShell.IShellFolder)
' This does not throw an error, but I have no clue what actually is happening.
' Marshal.GetObjectForIUnknown, as the name implies, should return an object which
' encapsulates an IUnknown interface, but I am giving a pointer to an IShellFolder
' interface, so I really wonder why it doesn't freak out. The original idea
' to try it that way was that I believe that IShellFolder inherits IUnknown,
' so it *eventually* could work that way. Could somebody comment about that?
i_Result = ishellfolder_Parent.GetUIObjectOf(0, 1, arintptr_RelPidList, IID_IExtractIcon, ui_ReturnFlags, intptr_CurrentIExtractIcon)
' Now this is the point where I am completely puzzled. i_Result is still 0,
' which means success, but intptr_CurrentIExtractIcon is 0 as well, meaning
' that something has gone horribly wrong. According to my understanding of
' the MSDN documentation, GetUIObjectOf MUST NOT return S_OK if something
' has gone wrong; in other words, if it returns S_OK, then intptr_CurrentIExtractIcon
' MUST be set to a reasonable value. Could somebody explain what's happening?
' I have left out the rest of the function because it wouldn't make any sense
' to use the NULL pointer for any further action.
End Function
End Class
任何想法我做錯了什麼？從文檔中，我得到每隔文件夾項目（虛擬或不）的印象和文件公開IExtractIcon。
Marshal.GetTypedObjectForIUnknown（）。我懷疑當你不發佈可運行代碼時，有人會爲你調試它。 –
@HansPassant感謝您的回覆。我已經嘗試過這種方法（而不是我上面顯示的），但也無法獲得IExtractIcon接口。同時，我認爲我發現了我的錯誤（請參閱下面的答案）。你認爲這個問題和答案是有用的嗎？如果沒有，我會刪除整個事情... – Binarus