Question

Hi I writing a wrapper in c# and i have some problem. I have this struct in c++.

typedef struct pjmedia_format
{
    pj_uint32_t id;
    pjmedia_type type;
    pjmedia_format_detail_type detail_type;
    union
    {
    pjmedia_audio_format_detail aud;
    pjmedia_video_format_detail vid;
    char user[PJMEDIA_FORMAT_DETAIL_USER_SIZE];
    } det;
} pjmedia_format;

This is link to this struct pjmedia_format

in c# I have this:

[StructLayout(LayoutKind.Sequential)]
public struct pjmedia_format
{
    public uint id;
    public pjmedia_type type;
    public pjmedia_format_detail_type detail_type;
    public det_t det;
}

[StructLayout(LayoutKind.Explicit)]
public struct det_t
{
    [FieldOffset(0)]
    [MarshalAs(UnmanagedType.Struct)]
    public pjmedia_audio_format_detail aud;
    [FieldOffset(36)]
    [MarshalAs(UnmanagedType.Struct)]
    public pjmedia_video_format_detail vid;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 1)]
    [FieldOffset(60)]
    public char[] user;
}

[StructLayout(LayoutKind.Sequential)]
public struct pjmedia_audio_format_detail
{
    public uint clock_rate;
    public uint channel_count;
    public uint frame_time_usec;
    public uint bits_per_sample;
    public int avg_bps;
    public int max_bps;
}

[StructLayout(LayoutKind.Sequential)]
public struct pjmedia_video_format_detail
{
    public pjmedia_rect_size size;
    public pjmedia_ratio fps;
    public int avg_bps;
    public int max_bps;
}

and when i want to use this struct i get this error

System.Runtime.InteropServices.MarshalDirectiveException was unhandled.
Message=”A method signature is not PInvoke compatible with the element.”

I try to use some attributes like size or pack but it doesn’t help (probably i use it wrong). I tested singly other struct e.g. pjmedia_video_format_detail and they works well. Any advice?

Best regards
Andrzej

Answer 1

As this is a union, shouldn’t that be:

[StructLayout(LayoutKind.Explicit)]
public struct det_t
{
    [FieldOffset(0)]
    [MarshalAs(UnmanagedType.Struct)]
    public pjmedia_audio_format_detail aud;
    [FieldOffset(0)]
    [MarshalAs(UnmanagedType.Struct)]
    public pjmedia_video_format_detail vid;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 1)]
    [FieldOffset(0)]
    public char[] user;
}

i.e. overlapping? Also, you might need user to be a fixed buffer rather than an array.