Re: [Swig-user] [SWIG csharp] Structure pointer-to-pointer in C#

[William S F. <ws...@fu...>]

Clinton Chau wrote:
> Hi all,
> 
> I am using SWIG to wrap a C library for use in some C#
> programs. I have run into the following problem: how
> do I wrap calls into the C library that allocate
> memory for structs?
> 
> I have studied the example that is provided in the
> SWIG documentation for the analogous problem in Java
> (section 19.9.10 in the 1.3 documentation). The
> problem I have is, how can I pass back the C pointer
> to the structure that has been allocated by the C
> routine?
> 
> In JNI, there are facilities to allow the JNI code to
> directly access and modify the members of a particular
> instance of a class. Unfortunately, these facilities
> don't exist for the C# case.
> 
> Am I missing something really simple? 
> 
> I have contemplated using the [StructLayout...]
> attribute using the csattribute typemap, and also
> passing by reference a IntPtr object to hold the the C
> pointer and then replacing the swigCPtr object when
> the P/Invoke call returns. But there has to be a more
> elegant solution, right?
> 
> Thanks in advance for any aid you can provide.
> 

The equivalent to JNI code modifying member variables is to make a call 
back into the CLR so that you call C# code from C. You use a delegate 
mapped to a C callback to achieve this. Look at the default marshalling 
for char* if you want to see an example.

The documentation you refer to also shows how to return the C pointer to 
the structure using %extend and this approach will also work for C# code.

William