Question

How can I send a managed object to native function to use it?

void managed_function()
{
  Object^ obj = gcnew Object();

  void* ptr = obj ??? // How to convert Managed object to void*?

  unmanaged_function(ptr);
}

// The parameter type should be void* and I can not change the type.
// This function is native but it uses managed object. Because type of ptr could not be 
// Object^ I called it "Unmanaged Function".
void unmanaged_function(void* ptr)
{
  Object^ obj = ptr ??? // How to convert void* to Managed object?

  obj->SomeManagedMethods();
}

Answer 1

After googling, reading MSDN and try some codes, I found this method to pass a managed object to an unmanaged function.

These methods show how to convert Object^ to void* and convert void* to Object^.

using namespace System;
using namespace System::Runtime::InteropServices;

void managed_function() 
{ 
  Object^ obj = gcnew Object();

  // Convert Object^ to void*
  GCHandle handle = GCHandle::Alloc(obj);
  IntPtr pointer = GCHandle::ToIntPtr(handle);
  void* ptr = pointer.ToPointer();

  unmanaged_function(ptr);

  handle.Free();
} 

void unmanaged_function(void* ptr) 
{
  // Convert void* to Object^
  IntPtr pointer(ptr);
  GCHandle handle = GCHandle::FromIntPtr(pointer);
  Object^ obj = (Object^)handle.Target;

  obj->SomeManagedMethods();
} 

Note: if “unmanaged_function” has variable arguments, this method won’t work.