Help with interop service, marshalling

I'm trying to call my DLL written in C, and got the error: "Cannot
marshal field 'b' of type 'mystruct': There is no marshaling support
for this type." The 2 functions in the DLL are:

void unpack(mystruct * str, void* in_buf);
void pack(mystruct* str, void* out_buf);

The data structure is as follows:

typedef struct
{
byte a;
} struct_a;

typedef struct
{
byte a;
byte b[15];
} struct_b;

typedef struct
{
byte a;
union
{
struct_a b;
struct_b c;
};
} struct_c;

typedef struct
{
byte a;
struct_c b[15];
struct_c c[15];
} mystruct;

My type definition in C# is:
public class PRLCodec
{
[DllImport("my.d ll")]
public extern static void unpack(
out mystruct str,
IntPtr buf);

[DllImport("my.d ll")]
public extern static void pack(
ref mystruct str,
IntPtr buf);
}

[StructLayout(La youtKind.Sequen tial)]
public struct struct_a
{
byte a;
}

[StructLayout(La youtKind.Sequen tial)]
public struct struct_b
{
byte a;
[MarshalAs(Unman agedType.ByValA rray, SizeConst=15)]
byte[] b;
}

[StructLayout(La youtKind.Explic it)]
public struct struct_c
{
[FieldOffset(0)]
byte a;
[FieldOffset(1)] // tried fieldoffset(4) as well for alignment
struct_a b;
[FieldOffset(1)]
struct_b c;
}

[StructLayout(La youtKind.Sequen tial)]
public struct mystruct
{
byte a;
[MarshalAs(Unman agedType.ByValA rray, SizeConst = 15)]
mystruct[] b;
[MarshalAs(Unman agedType.ByValA rray, SizeConst = 15)]
mystruct[] c;
}

The code compiles but gives the above error. What could be wrong? I
tried to read documents on interop service, but it seemed overwhelming
to a new .NET programmer. Any help is appreciated.