Passing a managed reference to a method taking unmanaged pointer

Question

Is it possible to make this work?

template<class T>
fun(T * t) { t->someMemberFunc(); }

... somewhere in the code:

ManagedType ^ managedP = gcnew ManagedType();
UnmanagedType * unmanagedP = new UnmanagedType();
fun(managedP);

Answer 1

You can't take the address of a managed object. The garbage collector can move it around in memory, invalidating the pointer value at any time. At a minimum you would have to pin the object first. Short from me not being able to come up with a valid syntax, pinning just to make a method call cannot be desirable. You'll need to declare the argument as a tracking handle:

template<typename T>
void fun(T^ t) { t->method(); }

Answer 2

This may be unrelated to your question, but GCHandle and gcroot may be useful; they allow you to get an unmanaged object encapsulating the handle to a managed object inside:

// hold_object_reference.cpp
// compile with: /clr
#include "gcroot.h"
using namespace System;

#pragma managed
class StringWrapper {

private:
   gcroot<String ^ > x;

public:
   StringWrapper() {
      String ^ str = gcnew String("ManagedString");
      x = str;
   }

   void PrintString() {
      String ^ targetStr = x;
      Console::WriteLine("StringWrapper::x == {0}", targetStr);
   }
};
#pragma unmanaged
int main() {
   StringWrapper s;
   s.PrintString();
}

-----------------

Imports System
Imports System.IO
Imports System.Threading
Imports System.Windows.Forms
Imports System.Runtime.InteropServices
Imports System.Security.Permissions

Public Delegate Function CallBack(ByVal handle As Integer, ByVal param As IntPtr) As Boolean

Module LibWrap

    ' passing managed object as LPARAM
    ' BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam);
    <DllImport("user32.dll")> _
    Function EnumWindows(ByVal cb As CallBack, ByVal param As IntPtr) As Boolean
    End Function
End Module 'LibWrap


Module App

    Sub Main()
        Run()
    End Sub

    <SecurityPermission(SecurityAction.Demand, UnmanagedCode:=true)> _
    Sub Run()

        Dim tw As TextWriter = System.Console.Out
        Dim gch As GCHandle = GCHandle.Alloc(tw)

        Dim cewp As CallBack
        cewp = AddressOf CaptureEnumWindowsProc

        ' platform invoke will prevent delegate to be garbage collected
        ' before call ends
        LibWrap.EnumWindows(cewp, GCHandle.ToIntPtr(gch))
        gch.Free()

    End Sub


    Function CaptureEnumWindowsProc(ByVal handle As Integer, ByVal param As IntPtr) As Boolean
        Dim gch As GCHandle = GCHandle.FromIntPtr(param)
        Dim tw As TextWriter = CType(gch.Target, TextWriter)
        tw.WriteLine(handle)
        Return True

    End Function
End Module