C# Callback Function Implementation

[Christopher Bartley]

Hi Everybody

I am having trouble wrapping some third party C software into C# in particular implementing the callback function. The function to be wrapped is of the form:

long setCallbackFunction(long fn(const HSIF_DATA_PT *pD1, const long N1, const HSIF_DATA_PT*pD2, const long N2));

where HSIF_DATA_PT is a data structure for storing points.

As expected without implementing any changes I get the type:

public int setCallbackFunction( SWIGTYPE_p_f_p_q_const__HSIF_DATA_PT_q_const__long_p_q_const__HSIF_DATA_PT_q_const__long__long fn )

which is not very useful.

I have tried using the swig callback functions in the interface file so that:

long setCallbackFunction( long (*op) (const HSIF_DATA_PT * pD1, const long N1, const HSIF_DATA_PT * pD2, const long N2) );

%callback("%callback _cb")

long fn(const HSIF_DATA_PT * pD1, const long N1, const HSIF_DATA_PT * pD2, const long N2)

%nocallback

...however this doesn't seem to get me anywhere either.

If anybody could help me with this problem or point me in the right direction any help would be much appreciated!

Thanks

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[David Piepgrass]

> I am having trouble wrapping some third party C software into C# in
> particular implementing the callback function. The function to be wrapped
> is of the form:
>
> long setCallbackFunction(long fn(const HSIF_DATA_PT *pD1, const long N1,
> const HSIF_DATA_PT*pD2, const long N2));
>
> where HSIF_DATA_PT is a data structure for storing points.
>
>
> As expected without implementing any changes I get the type:
>
> public int setCallbackFunction(
> SWIGTYPE_p_f_p_q_const__HSIF_DATA_PT_q_const__long_p_q_const__HSIF_DATA_PT
> _q_const__long__long fn )
>
>
> which is not very useful.
>
>
>
> I have tried using the swig callback functions in the interface file so
> that:
>
> long setCallbackFunction( long (*op) (const HSIF_DATA_PT * pD1, const long
> N1, const HSIF_DATA_PT * pD2, const long N2) );
>
> %callback("%callback _cb")
> long fn(const HSIF_DATA_PT * pD1, const long N1, const HSIF_DATA_PT * pD2,
> const long N2)
> %nocallback
>
> ...however this doesn't seem to get me anywhere either.

I don't know what %callback is, but I wrote a macro %cs_callback for this purpose:

///////////////////////////////////////////////////////////////////////////
// cs_callback is used to marshall callbacks. It allows a C# function to
// be passed to C++ as a function pointer through P/Invoke, which has the
// ability to make unmanaged-to-managed thunks. It does NOT allow you to
// pass C++ function pointers to C#.
//
// Anyway, to use this macro you need to declare the function pointer type
// TYPE in the appropriate header file (including the calling convention),
// declare a delegate named after CSTYPE in your C# project, and use this
// macro in your .i file. Here is an example:
//
// in C++ header file (%include this header in your .i file):
// typedef void (__stdcall *Callback)(PCWSTR);
// void Foo(Callback c);
//
// in C# code:
// public delegate void CppCallback([MarshalAs(UnmanagedType.LPWStr)]
//                                  string message);
//
// in your .i file:
// %cs_callback(Callback, CppCallback)
//
// As an alternative to specifying __stdcall on the C++ side, in the .NET
// Framework (but not the Compact Framework) you can use the following
// attribute on the C# delegate in order to get compatibility with the
// default calling convention of Visual C++ function pointers:
// [UnmanagedFunctionPointerAttribute(CallingConvention.Cdecl)]
//
// Remember to invoke %cs_callback BEFORE any code involving Callback.
%define %cs_callback(TYPE, CSTYPE)
        %typemap(ctype) TYPE, TYPE& "void*"
        %typemap(in) TYPE  %{ $1 = ($1_type)$input; %}
        %typemap(in) TYPE& %{ $1 = ($1_type)&$input; %}
        %typemap(imtype, out="IntPtr") TYPE, TYPE& "CSTYPE"
        %typemap(cstype, out="IntPtr") TYPE, TYPE& "CSTYPE"
        %typemap(csin) TYPE, TYPE& "$csinput"
%enddef


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