C# Marshalling for C Struct and Function

Question

I have the following C Header/Code Example:

Header file

struct category_info {
 int id;
 const char *name;
 const char *description;
};


DLLEXPORT
void* xyz_categories_info(struct category_info **info, size_t *info_count);

Example C Snippet

struct category_info *catinfo;

size_t catcount;
size_t i;
int max_name_len = 0;
void *catmem = xyz_categories_info(&catinfo, &catcount)

Which I would like to convert to c#...

My First GUESS (and its a guess) is:

[StructLayout(LayoutKind.Sequential)]
    public struct category_info
    {
        int id;
        [MarshalAs(UnmanagedType.LPStr)]
        StringBuilder name;
        [MarshalAs(UnmanagedType.LPStr)]
        StringBuilder description;
    };

[DllImport("mydll.dll", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr xyz_categories_info([Out]category_info cat, [Out]int catSize);

But it just doesn't look right..

Any suggestions.. Once the above is declared correctly in C#.. How should it be accessed in C#

category_info catinfo;

catmem = xyz_categories_info(out catinfo, out catcount);

??????

Any help greatly appreciated.

Thanks

================================================================================

Update 2

The memory allocated in xyz_categories_info is freed using this C call:

void xyz_categories_info_free(void *p);

Below is an example of it being used in C.... Hope this explains it a bit more..

    category_buffer = xyz_categories_info(&category_info, &category_count);

if( !category_buffer ) 
    {
    // Failed Log a message and exit.
    exit(1);
}

for(j=0; j<category_count; j++) 
    {
    if( category_info[j].id == 0 )
        continue;

    printf("id: %d name: '%s' description: '%s'\n",
        category_info[j].id,
        category_info[j].name,
        category_info[j].description
    );
}

xyz_categories_info_free(category_buffer);

What does xyz_categories_info exactly? Looking at its prototype, I can guess that it allocates array of category_info structures and places pointer to this array and its size to output parameter. What does it return? Your C code snippet doesn't contain this information. — Alex F
Please post full C code snippet which shows how information returned by this function is used and released. Using low-level Marshal functions and IntPtr type, we can write almost everything, that C does, though it is really much beter to do this in C++/CLI. — Alex F

Alex F Alex F · Accepted Answer · 2010-11-29T11:02:33

This code is compiled, but not tested. If you know C, you will understand what happens here, this is just the same C code translated to C#.

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    public struct category_info 
    { 
        public int id;
        public IntPtr name;
        public IntPtr description;
    }; 

    class Program
    {
        [DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr xyz_categories_info(ref IntPtr cat, ref int catSize);

        [DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern void xyz_categories_info_free(IntPtr cat); 

        static void Main(string[] args)
        {
            IntPtr categories = IntPtr.Zero;
            IntPtr category_buffer = IntPtr.Zero;
            int category_count = 0;
            category_info info = new category_info();
            IntPtr current;

            try
            {
                category_buffer = xyz_categories_info(ref categories, ref category_count);

                if (category_buffer == IntPtr.Zero)
                {
                    return;
                }

                if (category_count == 0)
                {
                    return;
                }

                for (int j = 0; j < category_count; j++)
                {
                    if (IntPtr.Size == 4)
                    {
                        current = new IntPtr(categories.ToInt32() + j * Marshal.SizeOf(info));
                    }
                    else
                    {
                        current = new IntPtr(categories.ToInt64() + j * Marshal.SizeOf(info));
                    }

                    info = (category_info)Marshal.PtrToStructure(current, typeof(category_info));

                    if (info.id == 0)
                    {
                        continue;
                    }

                    Console.WriteLine(info.id);
                    Console.WriteLine(Marshal.PtrToStringAnsi(info.name));
                    Console.WriteLine(Marshal.PtrToStringAnsi(info.description));
                }
            }
            finally
            {
                if (category_buffer != IntPtr.Zero)
                {
                    xyz_categories_info_free(category_buffer);
                }
            }
        }
    }
}

C# Marshalling for C Struct and Function

Update 2

2 Answers