Marshall Structs with ifdef's and unions in c#

Question

I have the following unmanaged code in C

typedef struct {         
PNIO_UINT16    AlarmSpecifier;
PNIO_UINT32    ModIdent;              
PNIO_UINT16    UserAlarmDataLen;    

#ifdef PNIO_ALARM_OLD_STRUC
    PNIO_UINT8     UserAlarmData[PNIO_MAX_ALARM_DATA_LEN]; 
#else
    union {
            PNIO_ALARM_DATA_MAINTENANCE_DIAGNOSIS m_diag; /* Another struct of size 20bytes */
            PNIO_UINT8     UserAlarmData[PNIO_MAX_ALARM_DATA_LEN]; /* Byte array, PNIO_MAX_ALARM_DATA_LEN=1472 */
          } UAData;
#endif

} ATTR_PACKED PNIO_ALARM_INFO;

Here's my managed conversion, I'm able to deduce sizes and layout the unions properly, but I don't know how to handle the ifdef condition from c code to c#

[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct PNIO_ALARM_INFO
{
    [FieldOffset(0)]
    public ushort AlarmSpecifier;

    [FieldOffset(2)]
    public uint ModIdent;

    [FieldOffset(6)]
    public ushort UserAlarmDataLen;

    // ifdef condition is true use this field
    //[FieldOffset(8)]
    //[MarshalAs(UnmanagedType.ByValArray, SizeConst = 1472)]
    //public byte[] UserAlarmData;

    // ifdef condition false use following fields
    [FieldOffset(8)]
    public PNIO_ALARM_DATA_MAINTENANCE_DIAGNOSIS m_diag;

    [FieldOffset(28)]
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 1472)]
    public byte[] UserAlarmData;
}

Can anyone tell me how to handle the if conditions and unions ?

Answer 1

Once compiled, structs in C and C# are static, there is no difference im this respect. #ifdef's also work pretty much the same way, no difference here either.

The only thing to realize here is that, based on the value of PNIO_ALARM_OLD_STRUC, the compilation can yield two different structs. You need to compile your C and C# code the same way, so that they yield the same struct definitions.

If you want to handle both C versions with one C# version, you need to define two different C# structs so that they both appear in one compilation unit. Then you need to figure out which struct to use when.