[Mono-dev] passing references to managed objects to unmanaged code

Jonathan Pryor jonpryor at vt.edu
Tue Feb 6 06:44:24 EST 2007

On Tue, 2007-02-06 at 12:00 +0100, Andreas Lagemann wrote:
> I am trying to pass a reference to a managed object to unmanaged code
> where I would like to handle it like a MonoObject, so I could use the
> momo_embed API to call methods of the managed object from unmanaged code.
> What I have in mind is some kind of C#/C++ peership where each C#
> instance of a certain Object has a C++ peer object and the other way round.

You might look at Gtk#, Qt#, Kimono, or
http://www.mono-project.com/Dllimport#Avoiding_Marshaling if you want a
peering arrangement.  This would work by having the C# types have an
IntPtr containing a pointer to the C++ object instance, and when you
P/Invoke your shared library you provide the C++ instance pointer.

> I tried something like this:
> C#:
> ...
> [DllImport("Simkernel",EntryPoint="SetCsharpPeer")]

However, if you want to use the Mono Embedding API, you can't use
[DllImport], you need to use
[MethodImplAttribute(MethodImplOptions.InternalCall)], e.g.:

	public extern void SetCsharpPeer (object o);

Doing this will require (1) that you embed Mono within your app, and (2)
call mono_add_internal_call ("YourType::SetCsharpPeer",

Note that you can't use C++ instance methods in this fashion, as Mono
won't know how to invoke C++ instance methods.  Static methods should
work like this.

Alternatively, you can look into Jon Chamber's COM Interop support for
Mono, which allows you to use COM interfaces to have C# code directly
invoke C++ code, even on Linux.

 - Jon

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