将包含 String 数组和 Integer 数组的结构传递到 C++ DLL

Question

我在将 VB.NET 编组到 C++ 时遇到问题，代码如下：

在 C++ DLL 中：

struct APP_PARAM
{
    int numData;
    LPCSTR *text;
    int *values;
};

int App::StartApp(APP_PARAM params)
{
    for (int i = 0; i < numLines; i++)
    {
        OutputDebugString(params.text[i]);
    }
}

在 VB.NET 中：

  <StructLayoutAttribute(LayoutKind.Sequential)> _
  Public Structure APP_PARAM
    Public numData As Integer
    Public text As System.IntPtr
    Public values As System.IntPtr
  End Structure

  Declare Function StartApp Lib "AppSupport.dll" (ByVal params As APP_PARAM) As Integer

  Sub Main()

    Dim params As APP_PARAM
    params.numData = 3

    Dim text As String() = {"A", "B", "C"}
    Dim textHandle As GCHandle = GCHandle.Alloc(text)
    params.text = GCHandle.ToIntPtr(textHandle)

    Dim values As Integer() = {10, 20, 30}
    Dim valuesHandle As GCHandle = GCHandle.Alloc(values)
    params.values = GCHandle.ToIntPtr(heightHandle)

    StartApp(params)

    textHandle.Free()
    valuesHandle.Free()

  End Sub

我检查了 C++ 端，OutputDebugString 的输出是垃圾，文本数组包含随机字符。这样做的正确方法是什么？

Answer 1

GCHandle.Alloc "分配普通句柄对于指定的对象”，“创建托管对象的句柄...这可以防止托管对象被收集”。

您正在寻找的是 System.Runtime.InteropServices.Marshal 中的方法，这些方法允许您执行诸如将托管对象复制到非托管代码可访问的内存之类的操作。不幸的是，根据 this，指针在你的结构中，它比许多其他东西更难封送（从某种意义上说，许多其他东西可以使用适当的 P/Invoke 属性自动封送），但这仍然是可能的。我已经尝试过这个方法并且它有效：

APP_PARAM param = new APP_PARAM();
string[] text = new string[] { "A", "B", "C" };
param.numData = text.Length;

// Manually allocate an array of pointers, one for each string.  arr holds the array's address.
IntPtr arr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(IntPtr)) * text.Length);
try
{
  param.text = arr;

  IntPtr[] unmanagedText = new IntPtr[text.Length];
  try
  {
    // Create a null-terminated ANSI string in unmanaged memory for each element in text.
    for (int i = 0; i < text.Length; i++)
      unmanagedText[i] = Marshal.StringToHGlobalAnsi(text[i]);
    // Copy the addresses of the unmanaged strings into the manually allocated array.
    // I don't know of any way to make an unmanaged copy of a managed array in one call.
    Marshal.Copy(unmanagedText, 0, arr, unmanagedText.Length);

    // param now looks like what the C++ code is expecting (except for the array of int).
    StartApp(param);
  }
  finally
  {
    foreach (IntPtr str in unmanagedText)
      Marshal.FreeHGlobal(str);
  }
}
finally
{
  Marshal.FreeHGlobal(arr);
}

您必须为 int 值数组拥有类似的分配/释放代码，并具有自己的 try/finally 块以确保调用 FreeHGlobal。

Answer 2

您需要使用 Marshal 类中的方法之一。

Dim str As String = "Hello World"
Dim ptr as IntPtr = Marshal.StringToHGlobalAnsi(str)
Try
  SomePInvokeCall(ptr)
Finally
  Marshal.FreeHGlobal(ptr)
End Try