VB.NET 二進制陣列傳遞給 C++ DLL Variant 參數的三種解決方案

問題核心

VB6 時代 Variant 直接支援 Byte() 陣列 (底層為 SAFEARRAY)，升級至 VB.NET 後 Object 類型封送為 Managed Array，C++ DLL 接收 VARIANT 期望 VT_ARRAY | VT_UI1，導致「類型不符」。

方案一：C++/CLI 封裝層 (最推薦)

建立 C++/CLI 專案做為中介層，將 Managed array^ 轉為 Native BYTE* + 長度呼叫原 DLL。

// NativeWrapper.h
#pragma once
using namespace System;

namespace NativeInterop {
    public ref class VariantBridge {
    public:
        // 假設原 DLL 導出: int SendData(const BYTE* data, int len);
        [DllImport("OriginalDll.dll", CallingConvention = CallingConvention::Cdecl)]
        static int SendDataNative(const unsigned char* data, int length);
        
        int SendData(array^ data) {
            if (data == nullptr || data->Length == 0) return -1;
            pin_ptr pinned = &data[0];
            return SendDataNative(pinned, data->Length);
        }
    };
}

VB.NET 呼叫：

Dim bridge As New NativeInterop.VariantBridge()
Dim data() As Byte = GetBinaryData()
Dim result As Integer = bridge.SendData(data)

方案二：修改 C++ DLL 導出介面 (若有原始碼)

新增不依賴 VARIANT 的導出函數：

// OriginalDll.cpp
extern "C" __declspec(dllexport) int SendDataByteArray(const BYTE* data, int length) {
    // 內部邏輯不變，直接用 data/length
    return InternalProcess(data, length);
}

// 保留舊介面相容
extern "C" __declspec(dllexport) int SendDataVariant(VARIANT data) {
    // 解析 VARIANT -> BYTE*/length 後呼叫 InternalProcess
    // ...
}

VB.NET 使用 DllImport 直接呼叫新介面：

Private Shared Function SendDataByteArray(data As Byte(), length As Integer) As Integer
End Function

' 呼叫
Dim result As Integer = SendDataByteArray(data, data.Length)

方案三：手動構造 VARIANT (無法修改 DLL 時)

使用 System.Runtime.InteropServices 手動建立 SAFEARRAY 並填入 VARIANT：

Imports System.Runtime.InteropServices

Module VariantHelper
    Private Const VT_UI1 As Short = &H11
    Private Const VT_ARRAY As Short = &H2000
    
    
    Private Structure SAFEARRAYBOUND
        Public cElements As Integer
        Public lLbound As Integer
    End Structure
    
    
    Private Structure VARIANT
        Public vt As Short
        Public wReserved1 As Short
        Public wReserved2 As Short
        Public wReserved3 As Short
        Public data As IntPtr
    End Structure
    
    
    Private Function SafeArrayCreate(vt As Short, cDims As Integer, bounds As SAFEARRAYBOUND()) As IntPtr
    End Function
    
    
    Private Function SafeArrayAccessData(psa As IntPtr, ByRef ppData As IntPtr) As Integer
    End Function
    
    
    Private Function SafeArrayUnaccessData(psa As IntPtr) As Integer
    End Function
    
    
    Private Sub SafeArrayDestroy(psa As IntPtr)
    End Sub
    
    Public Function CreateByteArrayVariant(data As Byte()) As Object
        If data Is Nothing OrElse data.Length = 0 Then Return Nothing
        
        Dim bounds As SAFEARRAYBOUND() = {New SAFEARRAYBOUND With {.cElements = data.Length, .lLbound = 0}}
        Dim psa As IntPtr = SafeArrayCreate(VT_UI1, 1, bounds)
        
        Try
            Dim pData As IntPtr
            SafeArrayAccessData(psa, pData)
            Marshal.Copy(data, 0, pData, data.Length)
            SafeArrayUnaccessData(psa)
            
            Dim var As New VARIANT With {.vt = VT_ARRAY Or VT_UI1, .data = psa}
            Return var  ' 傳遞給接受 Object (VARIANT) 的 COM 介面
        Catch
            SafeArrayDestroy(psa)
            Throw
        End Try
    End Function
End Module

' 使用
Dim var As Object = VariantHelper.CreateByteArrayVariant(binaryData)
comObject.SendData(var)  ' 假設 COM 介面方法簽名為 SendData(ByVal data As Object)

方案選擇決策樹

有 C++ DLL 原始碼？
  ├─ 是 → 能重新編譯部署 DLL？
  │       ├─ 是 → 方案二 (修改 DLL 介面) 最乾淨
  │       └─ 否 → 方案一 (C++/CLI 封裝) 無需動 DLL
  └─ 否 → 方案三 (手動 VARIANT) 或 方案一 (C++/CLI 包裝現有 DLL)

常見陷阱與除錯

最佳實務總結

1. 優先選擇方案一 (C++/CLI)：型別安全、效能佳、隔離 Native 複雜度、易於單元測試。
2. 避免方案三：除非極度受限環境，手動構造 VARIANT 維護成本極高且易出錯。
3. 長期規劃：逐步將 C++ DLL 介面現代化 (移除 VARIANT，改用純 C 介面或 COM)，配合 .NET Native AOT 或 C++/CLI 互通。