问题
I'm having problems with marshaling in VB.NET to C++, here's the code :
In the C++ DLL :
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]);
}
}
In VB.NET :
<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
I checked the C++ side, the output from the OutputDebugString is garbage, the text array contains random characters. What is the correct way to do this?
回答1:
GCHandle.Alloc
"Allocates a Normal handle for the specified object", which "creates a handle to a managed object ... which prevents the managed object from being collected".
What you're looking for is the methods from System.Runtime.InteropServices.Marshal
, which allow you to do things like copy managed objects to memory accessible by unmanaged code. Unfortunately, according to this, the pointers in your struct make it a little harder to marshal than many other things (in the sense that many other things can be automatically marshalled using the appropriate P/Invoke attributes), but it's still possible. I've tried this out and it works:
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);
}
You'll have to have similar allocation/free code for your array of int values, with its own try/finally blocks to make sure FreeHGlobal is called.
回答2:
You need to use one of the methods from the Marshal
class.
Dim str As String = "Hello World"
Dim ptr as IntPtr = Marshal.StringToHGlobalAnsi(str)
Try
SomePInvokeCall(ptr)
Finally
Marshal.FreeHGlobal(ptr)
End Try
来源:https://stackoverflow.com/questions/2919048/passing-a-structure-containing-an-array-of-string-and-an-array-of-integer-into-a