Interop Structure: Should Unsigned Short be Mapped to byte[]?

Question

I have such a C++ structure:

typedef struct _FILE_OP_BLOCK
{                                                                                                                          
    unsigned short fid;     // objective file ID 
    unsigned short offset;  // operating offset
    unsigned char len;      // buffer length(update)
                            // read length(read)        
    unsigned char buff[240];
} FILE_OP_BLOCK;

And now I want to map it in .Net. The tricky thing is that the I should pass a 2 byte array for fid, and integer for len, even though in C# fid is an unsigned short and len is an unsigned char

I wonder whether my structure ( in C#) below is correct?

   [StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Auto)]
   public struct File_OP_Block 
    {
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
        public byte[] fid;

        public ushort offset;

        public byte length;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 240)]
        public char[] buff;
    }

Answer 1

Given that the C++ short data type is actually two bytes, a two byte array should work. The integer sizes in C/C++ are not strictly defined, so the standard only says that a short is at least two bytes.

The C# char data type is a 16 bit unicode character, so that doesn't match the C++ char data type which is an 8 bit data type. You either need an attribute to specify how the characters are encoded into bytes, or use a byte array.

You might need an attribute to specify the packing, so that there is no padding between the members.

Answer 2

Your CharSet property on the [DllImport] attribute is definitely wrong, you need CharSet.Ansi to get the P/Invoke marshaller to convert it to a char[]. Declare the buff member as a string for easier usage. While declaring the fid member as a byte[] isn't wrong, I really don't see the point of it. That the unmanaged code copies a char[] into it is an implementation detail. Thus:

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Ansi)]
public struct File_OP_Block 
{
    public ushort fid;
    public ushort offset;
    public byte length;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 240)]
    public string buff;
}