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(); }

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