c# - ユニオンを使用して C 構造体を C# コードにマーシャリングするときにガベージ データを取得する

Question

マネージド アプリケーションで次の C 構造体を使用しようとしています

typedef struct libvlc_media_track_t
{
    uint32_t    i_codec;
    uint32_t    i_original_fourcc;
    int         i_id;
    libvlc_track_type_t i_type;

    int         i_profile;
    int         i_level;

    union {
        libvlc_audio_track_t *audio;
        libvlc_video_track_t *video;
        libvlc_subtitle_track_t *subtitle;
    };

    unsigned int i_bitrate;
    char *psz_language;
    char *psz_description;

} libvlc_media_track_t;

typedef struct libvlc_audio_track_t
    {
        unsigned    i_channels;
        unsigned    i_rate;
    } libvlc_audio_track_t;

    typedef struct libvlc_video_track_t
    {
        unsigned    i_height;
        unsigned    i_width;
        unsigned    i_sar_num;
        unsigned    i_sar_den;
        unsigned    i_frame_rate_num;
        unsigned    i_frame_rate_den;
    } libvlc_video_track_t;

    typedef struct libvlc_subtitle_track_t
    {
        char *psz_encoding;
    } libvlc_subtitle_track_t;

unsigned libvlc_media_tracks_get( libvlc_media_t *p_md, libvlc_media_track_t ***tracks );

.NET バージョンは次のようになります。

[StructLayout(LayoutKind.Explicit)]
public struct libvlc_media_track_t
{
    [FieldOffset(0)]
    public uint i_codec;
    [FieldOffset(4)]
    public uint i_original_fourcc;
    [FieldOffset(8)]
    public int i_id;
    [FieldOffset(12)]
    public libvlc_track_type_t i_type;
    [FieldOffset(16)]
    public int i_profile;
    [FieldOffset(20)]
    public int i_level;
    [FieldOffset(24)]
    public libvlc_audio_track_t audio;
    [FieldOffset(24)]
    public libvlc_video_track_t video;
    [FieldOffset(24)]
    public libvlc_subtitle_track_t subtitle;
    [FieldOffset(48)]
    public uint i_bitrate;
    [FieldOffset(52)]
    public IntPtr psz_language;
    [FieldOffset(56)]
    public IntPtr psz_description;
}

[StructLayout(LayoutKind.Sequential)]
public struct libvlc_audio_track_t
{
    public uint i_channels;
    public uint i_rate;
}

[StructLayout(LayoutKind.Sequential)]
public struct libvlc_video_track_t
{
    public uint i_height;
    public uint i_width;
    public uint i_sar_num;
    public uint i_sar_den;
    public uint i_frame_rate_num;
    public uint i_frame_rate_den;
}

[StructLayout(LayoutKind.Sequential)]
public struct libvlc_subtitle_track_t
{
    public IntPtr psz_encoding;
}

[DllImport("libvlc", CallingConvention = CallingConvention.Cdecl)]
public static unsafe extern int libvlc_media_tracks_get(IntPtr media, libvlc_media_track_t*** ppTracks);

unsafe
{
     libvlc_media_track_t** ppTracks;
     int num = LibVlcMethods.libvlc_media_tracks_get(m_hMedia, &ppTracks);
     if (num == 0 || ppTracks == null)
     {
         throw new Exception();
     }

     for (int i = 0; i < num; i++)
     {
         libvlc_media_track_t* pTrackInfo = ppTracks[i];            
     }

     LibVlcMethods.libvlc_media_tracks_release(ppTracks, num);
}

コードは例外なく動作しますが、ほとんどの構造体/共用体フィールドでガベージ データを取得します。

アドバイスしてください、事前に感謝します

編集：以下も試しましたが、同じ結果でした

[StructLayout(LayoutKind.Sequential)]
    public struct libvlc_media_track_t
    {
        public uint i_codec;
        public uint i_original_fourcc;
        public int i_id;
        public libvlc_track_type_t i_type;
        public int i_profile;
        public int i_level;
        public MediaVariant media;
        public uint i_bitrate;
        public IntPtr psz_language;
        public IntPtr psz_description;
    }

[StructLayout(LayoutKind.Explicit)]
public struct MediaVariant
{
    [FieldOffset(0)]
    public libvlc_audio_track_t audio;

    [FieldOffset(0)]
    public libvlc_video_track_t video;

    [FieldOffset(0)]
    public libvlc_subtitle_track_t subtitle;
}