[Mono-dev] C++ to C# to C++ interop, how can I do this in a mono-compliant way?

[William S Fulton]

Miguel de Icaza wrote:
> Hello,
>  
>>Is there any way to have this work under Mono?   Please realize that
>>the basic need is to have 1 instance of a C++ exe call into a .NET
>>dll, and have that DLL be able to then execute functions in the C++
>>exe that called it.     So this requires a mono-equivlant of PInvoke,
>>plus a way to have the C++ app call the C# app. 
>> 
>>Help on this would be appreciated, otherwise it'll be Windows only!
> 
> If you do not mind writing some bridge code, just use P/Invoke on the C#
> side to call your C++ code.
> 
> Notice that all the entry points that you will access have to be:
> 
> 	extern "C" { .... }
> 
> So that P/Invoke can locate them.
> 
> For calling from C++ to managed land, you can use function pointers:
> register a delegate as a function pointer with your C++ code and have
> that be your gateway to communicate back:
> 
> file.c:
> 	typedef void (*mono_print_hello_fn) ();
> 	mono_print_hello_fn printer;
> 
> 	routine ()
> 	{
> 		(*printer) ();
> 	}
> 
> 	set_hello_func (mono_print_hello_fn f)
> 	{
> 		printer = f;
> 	}
> 
>>From C# you call:
> 
> 	delegate void MyPrinter ();
> 	[DllImport (...)]
> 	extern static int set_hello_func (MyPrinter f);
> 
> 	Main ()
> 	{
> 		set_hello_func (new MyPrinter (func));
> 		
> 	}
> 
> 	void MyPrinter ()
> 	{
> 		Console.WriteLine ("Hello");
> 	}

If you have many C++ functions to call from C#, consider using SWIG 
http://www.swig.org to automate the generation of the PInvoke bridge 
code. SWIG requires your C++ header files as input and generates C# 
proxy classes for your C++ classes. The tool doesn't yet wrap callbacks 
from C++ to C# using function pointers/delegates so you'll have to write 
this by hand as shown above.

William