PInvoke FormatMessage 消息 ByRef lpBuffer As [String] Is Nothing

Question

正在声明此函数：

    <DllImport("kernel32.dll", EntryPoint:="FormatMessageW", SetLastError:=True, CharSet:=CharSet.Unicode, CallingConvention:=CallingConvention.StdCall)>
Public Shared Function FormatMessage(ByVal dwFlags As Integer,
                                     ByRef lpSource As IntPtr,
                                     ByVal dwMessageId As Integer,
                                     ByVal dwLanguageId As Integer,
                                     ByRef lpBuffer As [String],
                                     ByVal nSize As Integer,
                                     ByRef Arguments As IntPtr) As Integer
End Function

基于此处的定义：https://pinvoke.net/default.aspx/kernel32/FormatMessage.html

场景
我目前正在 PInvoking wininet.dll 对服务器执行 FTP 事务。如果遇到错误，我会从 Err.LastDllError 返回错误代码。我定义了一个函数，它获取 dll 错误和 returns 基于错误代码的消息，但是它没有按预期工作。这是我用来抛出 dll 错误的函数：

Private Sub ThrowLastDllError()
    Dim iLastErrorID As Integer = Err.LastDllError
    Dim iMessageBuffer As IntPtr = Nothing
    Dim iModuleHandle As IntPtr = GetModuleHandle("wininet.dll")

    Dim sMessageBuffer As String = Nothing

    If iLastErrorID > 12000 And iLastErrorID < 12157 Then
        FormatMessage(FORMAT_MESSAGE_FROM_HMODULE Or
                     FORMAT_MESSAGE_IGNORE_INSERTS Or
                     FORMAT_MESSAGE_ALLOCATE_BUFFER,
                     iModuleHandle,
                     iLastErrorID,
                     0,
                     sMessageBuffer,
                     256,
                     Nothing)

        Debugger.Break()
        'TODO: Throw exception with error code message here
    End If
End Sub

基于此处描述的技术：https://docs.microsoft.com/en-us/windows/desktop/wininet/appendix-c-handling-errors I am expecting to get some kind of string message based on the error code for this particular dll, for example if I get an error code of 12110 (ERROR_FTP_TRANSFER_IN_PROGRESS. Reference: https://support.microsoft.com/en-au/help/193625/info-wininet-error-codes-12001-through-12156) 如果不相同 "The requested operation cannot be made on the FTP session handle because an operation is already in progress."，我希望得到类似于以下内容的消息（在变量 sMessageBuffer 中）。然而，sMessageBuffer 从未被赋值并且保持为空。我只能假设我以某种方式滥用了这种技术，我尝试了在线论坛和本网站本身描述的各种方法，但我没有成功。

Answer 1

这里有一些有效的示例代码：

Sub Main()

    Dim h As IntPtr = LoadLibrary("wininet.dll") ' or GetModuleHandle ...
    Dim sb = New StringBuilder(1024)
    FormatMessage(Format_Message.FORMAT_MESSAGE_FROM_HMODULE Or Format_Message.FORMAT_MESSAGE_IGNORE_INSERTS,
        h,
        12002,
        0,
        sb,
        sb.Capacity,
        Nothing)

    Console.WriteLine(sb) ' prints "The operation timed out"
   ' FreeLibrary, etc.
End Sub

Enum Format_Message
    FORMAT_MESSAGE_IGNORE_INSERTS = &H200
    FORMAT_MESSAGE_FROM_SYSTEM = &H1000
    FORMAT_MESSAGE_FROM_HMODULE = &H800
End Enum

<DllImport("Kernel32", SetLastError:=True, CharSet:=CharSet.Unicode)>
Public Function FormatMessage(ByVal dwFlags As Format_Message, ByVal lpSource As IntPtr, ByVal dwMessageId As Integer, ByVal dwLanguageId As Integer, lpBuffer As StringBuilder, ByVal nSize As Integer, ByVal Arguments As IntPtr) As Integer
End Function

<DllImport("kernel32", SetLastError:=True, CharSet:=CharSet.Unicode)>
Public Function LoadLibrary(ByVal lpFileName As String) As IntPtr
End Function

PInvoke FormatMessage 消息 ByRef lpBuffer As [String] Is Nothing

PInvoke FormatMessage Message ByRef lpBuffer As [String] Is Nothing

vb.net

pinvoke

wininet

kernel32